Introduction 
to 


Optical    Mineralogy 
and  Petrography 


M.  G.  EDWARDS 


Louis  Byrne 

Slichter 


INTRODUCTION 
TO 

Optical  Mineralogy 
and  Petrography 

The  Practical  Methods  of 

Identifying  Minerals 

in   Thin   Section 

With    the 

Microscope 

and 

The  Principles  Involved  in 
The  Classification  of  Rocks 


By 
M.  G.  EDWARDS,  A.M. 

Instructor  in  Geology  and  Mineralogy 
Case  School  of  Applied  Science. 


CLEVELAND,  OHIO, 
1916. 


Copyright,  191G,  by 
M.  G.  EDWARDS 


The  Gardner  Printing   Co. 

Cleveland 

1916 


<3eo/. 
Ub. 


PREFACE. 

IN  THE  preparation  of  this  volume  the  writer  has  at- 
tempted to  gather  together  and  systematize  in  a  manner 
accessible  for  ready  reference  those  facts  which  are  essen- 
tial to  a  field  geologist  or  to  a  mining  engineer  in  an 
understanding  of  the  fundamental  principles  involved  in 
the  classification  and  identification  of  rocks.  In  the  field, 
a  preliminary  classification  is  usually  made  by  macro- 
scopic means.  However,  it  is  often  necessary  to  make  a 
more  careful  classification  by  a  microscopic  examination 
of  a  thin  section  of  the  minerals  comprising  the  rock 
mass.  To  do  this  successfully  requires  a  knowledge  of 
the  application  of  light  to  crystalline  substances. 

This  volume  differs  from  most  of  the  reference  and 
text  books  relating  to  this  subject  in  that  it  incorporates 
in  one  volume  the  elements  of  optical  mineralogy  and  the 
elements  of  petrography.  In  Part  One,  eight  general 
operations  for  the  determination  of  unknown  minerals  in 
thin  section  are  described,  prefaced  by  a  short  summary 
of  the  principles  of  optics  which  apply  to  the  transmission 
of  polarized  light  through  minerals.  Descriptions  of  fifty- 
eight  of  the  most  common  of  the  rock-making  minerals  are 
given,  special  attention  being  given  to  the  criteria  for 
the  determination  of  these  minerals  in  thin  section. 
Their  form,  cleavage,  twinning,  color,  refringence,  bi- 
refringence, extinction  angles,  pleochroism,  absorption, 
optical  character,  inclusions,  alterations,  occurrences, 
uses,  and  differentiation  from  similar  minerals,  are  all 
discussed  whenever  applicable.  An  elementary  knowl- 
edge of  crystallography  and  descriptive  mineralogy  is 
assumed. 


4  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

In  Part  Two,  the  principles  of  petrography  are  dis- 
cussed briefly.  Attention  is  given  to  the  classification 
and  description  of  the  more  important  igneous  rock 
types. 

Following  Iddings,  Winchell,  and  other  American 
petrographers,  the  symbols  X,  Y,  and  Z,  are  here  em- 
ployed in  referring  to  the  axes  of  ether  elasticity,  instead 
of  the  German  a,  b,  and  c,  used  in  many  text  and  refer- 
ence books.  This  is  done  to  avoid  confusion,  especially 
in  conversation  or  discussion,  with  the  crystallographic 
axes. 

The  writer  is  indebted  to  Professor  Frank  R.  Van 
Horn  for  suggestions.  Among  the  reference  and  text 
books  most  frequently  consulted  the  writer  wishes  to 
acknowledge  Winchell's  "Elements  of  Optical  Miner- 
alogy," Johannsen's  "Manual  of  Petrographic  Methods," 
Luquer's  "Minerals  in  Rock  Sections,"  Rogers's  "Study  of 
Minerals,"  Findlay's  "Igneous  Rocks,"  Kemp's  "Hand- 
book of  Rocks,"  Ries'  and  Watson's  "Engineering  Geol- 
ogy," and  Farrell's  "Practical  Field  Geology." 

M.  G.  EDWARDS. 
Cleveland,  Ohio,  February,  1916. 


TABLE    OF    CONTENTS 

INTRODUCTION.  PAGE 

PART  ONE.  — OPTICAL  MINERALOGY. 

CHAPTER  1.  —  THE  ELEMENTS  OF  OPTICS  AND  THE 
APPLICATION  OF  POLARIZED  LIGHT  TO  CRYSTALLINE 

SUBSTANCES   13 

The  Nature  of  Light  —  Isotropic  and  Aniso- 
tropic  Media  —  Uniaxial  and  Biaxial  Crystals 

—  Index  of  Refraction  —  Double  Refraction  — 
Interference  —  Polarization. 

CHAPTER  2.  —  THE  POLARIZING  MICROSCOPE  AND  ITS 
PARTS   25 

Microscope  —  Nicol  Prisms  —  Condensing  Lens 

—  Cross  Hairs  —  Stage  —  Mirror  —  Objective 
—  Bertrand  Lens  —  Ocular  Micrometer  —  Ad- 

justment  Screws. 

CHAPTER  3.  —  GENERAL  METHODS  OF  MINERAL  DE- 
TERMINATION         33 

1.  By  the  General  Physical  Properties;  2.  By 
the  Relative  Refractive  Index  —  Method  of  Due 
de  Chaulnes  —  Immersion  Method  —  Becke 
Method  —  Scale  of  Refringence ;  3.  By  the  Bire- 
fringence —  Interference  Colors  —  Axes  of 
Ether  Vibration  —  Optic  Plane  —  Scale  of  Bi- 
refringence. 

CHAPTER  4.  —  GENERAL  METHODS  OF  MINERAL  DE- 
TERMINATION (Continued)  51 

4.    By   Axial   Interference   Figures  —  Uniaxial 


6  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

and  Biaxial;  5.  By  Dispersion;  6.  By  Optical 
Character  —  Quartz-Sensitive  Tint  —  Quarter- 
Undulation  Mica  Plate  —  Quartz  Wedge ;  7.  By 
the  Extinction  Angle  —  Parallel  and  Oblique 
Extinction;  8.  By  Pleochroism. 

CHAPTER  5.  —  DESCRIPTION  OF  IMPORTANT  ROCK- 
MAKING  MINERALS  69 

Isometric  Minerals. 

CHAPTER    6.  —  DESCRIPTION    OF    MINERALS    (Con- 
tinued)         75 

Tetragonal  —  Hexagonal. 

CHAPTER    7. —  DESCRIPTION    OF    MINERALS    (Con- 
tinued)       86 

Orthorhombic  —  Monoclinic  —  Triclinic. 

PART  TWO.  — PETROGRAPHY. 

CHAPTER   8.  —  GENERAL    DISCUSSION    OF   IGNEOUS 

ROCKS   123 

Classification  —  Essential  and  Accessory  Min- 
erals—  Primary  and  Secondary  Minerals  — 
Texture  —  Rosenbusch's  Law  —  Volcanic  and 
Plutonic  Rocks  —  Petrogeny  —  Magmas  —  Dif- 
ferentiation —  Magmatic  Stoping  —  Crystal- 
lization—  Influence  of  Gases  on  a  Magma  — 
Relation  between  Composition  of  Igneous  Rocks 
and  Magmas  —  Aids  in  the  Determination  of 
Igneous  Rocks  in  Hand  Specimen. 

CHAPTER   9.  —  IGNEOUS   ROCK    TYPES  —  PLUTONIC 

ROCKS 138 

Granite  —  Syenite  —  Nephelite  and  Leucite 
Syenite  —  Diorite  —  Gabbro  and  Norite  —  Es- 


TABLE   OF  CONTENTS  7 

sexite  —  Theralite,  Shonkinite,  Malignite,  Ijol- 
ite,  Missourite  —  Peridotite  —  Pyroxenite, 
Hornblendite. 

CHAPTER  10.  —  IGNEOUS  ROCK  TYPES  —  VOLCANIC 

ROCKS ; 158 

Rhyolite  —  Trachyte  —  Phonolite  —  Andesite 
Dacite  —  Basalt  —  Trachydolerite  —  Tephrite, 
Basanite  —  Leucitite,  Nephelinite  —  Limburg- 
ite  —  Augitite.  Pyroclastic  rocks. 

CHAPTER    11.  —  SEDIMENTARY    AND    METAMORPHIC 

ROCKS 172 

Sedimentary  Rocks  —  Classification — Conglom- 
erate —  Breccia  —  Sandstone  —  Shale  —  Loess 

—  Sand    Dunes  —  Limestone  —  Gypsum  —  An- 
hydrite —  Halite  —  Flint  —  Iron   Ores  —  Phos- 
phate Rock  —  Carbonaceous  Rock. 
Metamorphic    Rocks  —  Composition,    Chemical 
and  Mineralogical  —  Agents  of  Metamorphism 

—  Gneiss  —  Schist  —  Quartzite  —  Slate    and 
Phyllite  —  Marble  —  Serpentine  —  Ophicalcite 

—  Soapstone. 

APPENDIX. 

SUGGESTIONS  FOR  GEOLOGICAL  WORK 185 

Observation  for  Geological  Mapping.  Criteria 
of  Relative  Age.  Table  for  the  Examination  of 
Rocks  in  the  Laboratory. 

INDEX  .  .  193 


INTRODUCTION. 

THE  TERM  Petrology  is  derived  from  the  two  Greek 
words  petros  (rock)  and  logos  (discourse),  from  which 
the  modern  definition,  the  science  or  treatise  of  rocks,  has 
been  evolved.  The  term  has  a  wide  scope,  and  embraces 
not  only  the  study  of  the  origin  and  transformation  of 
rocks  but  a  consideration  of  their  mineral  composition, 
classification,  description  and  identification  based  upon 
either  megascopic  or  microscopic  characteristics. 

Petrology  may  be  subdivided  into  the  following  spe- 
cial studies : 

Petrogeny,  which  is  concerned  with  the  origin  of 
rocks,  and 

Petrography,  which  is  concerned  with  the  systematic 
classification  and  description  of  rocks  megascopically  and 
microscopically.  It  is  the  latter  phase  of  the  subject 
which  is  dealt  with  chiefly  in  the  following  notes. 

Petrography  may  be  divided  for  the  sake  of  con- 
venience into  megascopic  petrography  and  microscopic 
petrography,  depending  upon  whether  or  not  the  student 
is  basing  his  identification,  classification  and  description 
upon  a  study  of  the  rock  in  hand  specimen  or  in  thin  sec- 
tion with  the  aid  of  the  polarizing  microscope. 

The  use  of  the  polarizing  microscope  necessarily  en- 
tails a  brief  review  of  the  elements  of  optics  and  a  con- 
sideration of  the  application  of  polarizing  light  to  crys- 
talline substances.  This  is  a  special  study  in  itself,  and 
is  called  Optical  Mineralogy.  Assuming  that  the  student 
has  had  little  or  no  previous  experience  with  the  study 
of  the  optical  properties  of  minerals,  a  short  review  of 
the  optical  characters  of  the  more  important  rock-making 


10  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

minerals  is  given.  Special  attention  is  given  to  the 
criteria  for  the  determination  of  the  mineral  in  thin  sec- 
tion and  diagnostics  for  the  differentiation  of  the  mineral 
from  similar  minerals. 

History  of  Petrography. — Great  advances  in  the  knowl- 
edge of  mineralogy  marked  the  latter  half  of  the  eight- 
eenth century.  Incidentally  there  followed  several  at- 
tempts to  classify  rocks,  which  resulted  in  1787  in  the 
publication  of  two  classifications  by  Karl  Haidinger 
(Vienna)  and  A.  G.  Werner  (Dresden).  Werner's  classi- 
fication was  stratigraphic  rather  than  petrographic,  but 
he  described  rocks  in  terms  of  mineralogical  composition 
and  physical  characteristics,  and  he  differentiated  be- 
tween essential  and  accessory  minerals. 

In  1801,  Abbe  R.  J.  Hauy  (Paris),  a  mineralogist, 
published  the  first  systematic  classification,  and  his 
"Traite  de  mineralogie"  with  subsequent  revisions  re- 
mained a  classic  for  a  long  period.  He  distinguished  five 
classes  of  rocks:  stony  and  saline,  combustible  nonme- 
tallic,  metallic,  rocks  of  an  igneous  or  aqueous  origin, 
and  volcanic  rocks. 

John  Pinkerton  (England)  in  1811  published  a  "Pe- 
trology, a  Treatise  on  Rocks,"  of  1200  pages.  In  view  of 
the  fact  that  natural  history  was  divided  into  three  king- 
doms— the  animal,  vegetable,  and  mineral — he  believed 
it  the  most  natural  classification  to  subdivide  the  mineral 
kingdom  into  provinces  and  domains.  Accordingly  he 
introduced  the  following  three  provinces :  Petrology,  the 
knowledge  of  rocks  or  stones  in  large  masses ;  Lithology, 
the  knowledge  of  gems  and  small  stones,  and  Metallogy, 
the  knowledge  of  metals.  Pinkerton's  volume  was  lightly 
regarded  even  by  his  contemporaries. 

Cordier  (France)  in  1815  classified  rocks  as  feld- 
spathic  or  pyroxenic,  and  made  subdivisions  according  to 
texture. 


INTRODUCTION  11 

Karl  von  Leonhard  (Heidelberg)  in  1823  and  Alex- 
andre  Brongniart  in  1827  proposed  systems  which  mark 
the  real  origin  of  systematic  petrography.  Mineral  com- 
position was  the  chief  factor  in  the  classification.  The 
former  established  four  divisions :  heterogeneous  rocks, 
homogeneous  rocks,  fragmental  rocks,  and  loose  rocks. 
The  latter  made  only  two  classes:  homogeneous  rocks 
and  heterogeneous  rocks. 

Hermann  Abich  in  1841  made  a  classification  of  the 
eruptive  rocks  according  to  the  content  of  the  various 
feldspars. 

The  term  petrography  was  perhaps  first  used  by  Carl 
Friedrich  Naumann,  who  in  1850  published  his  "Lehr- 
buch  der  Geognosie,"  in  which  he  divided  all  rock  classes 
into  crystalline  rocks,  clastic  rocks,  and  rocks  which  are 
neither  crystalline  nor  clastic.  In  a  later  revision  he 
recognized  only  two  classes:  the  original,  and  the  de- 
rived. 

Several  classifications  were  presented  in  the  next  few 
decades  by  von  Gotta  (1855),  Senft  (1857),  Blum 
(1860),  Roth  (1861),  Scheerer  (1864),  Ferdinand  Zirkel 
(1866),  and  F.  von  Richthofen  (1868),  based  upon  min- 
eral constitution,  chemical  composition,  structure,  and 
texture,  with  an  increasing  tendency  to  emphasize  the 
importance  of  mineral  composition. 

A  new  era  in  the  development  of  petrography  dawned 
with  the  introduction  of  the  polarizing  microscope.  With 
the  greater  knowledge  of  mineral  composition  and  texture 
thus  revealed,  the  old  schemes  were  discarded  or  radically 
revised,  new  terms  introduced,  and  the  nomenclature 
became  rapidly  more  complex.  Although  Henry  Clifton 
Sorbey  (England)  perhaps  first  used  the  microscope  in 
the  determination  of  rock  sections,  it  was  not  until  the 
decade  between  1870  and  1880  that  microscopic  methods 
began  to  exert  a  controlling  influence  in  the  development 


12  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

of  the  science.  Zirkel  in  1873  produced  "Die  mikroskop- 
ische  Beschaffenheit  der  Mineralien  und  Gesteine,"  which 
shows  a  remarkable  and  significant  advance  in  the  prog- 
ress of  petrography  in  the  eight  years  following  the  pub- 
lication of  his  "Lehrbuch."  He  dealt  chiefly  with  feld- 
spathic,  massive,  composite,  and  nonclastic  rocks. 

In  France  in  1879  the  "Mineralogie  micrographique," 
by  F.  Fouque  and  A.  Michel  Levy,  appeared.  Rock  classi- 
fication was  based  upon  the  mode  of  formation,  the  geo- 
logical age,  and  the  specific  mineral  properties,  which 
includes  the  nature  of  the  mineral  and  the  structure  of 
the  rock. 

Subsequent  editions  of  the  original  works  of  Rosen- 
busch  and  Zirkel,  and  a  number  of  new  noteworthy  con- 
tributions by  Roth  (1883),  Teall  (1888),  Loewinson- 
Lessing  (1890-1897),  and  Johannes  Walther  (1897) 
appeared,  chief  attention  being  given  to  the  classification 
of  igneous  rocks  on  the  basis  of  origin,  age,  and  char- 
acters. 

"\Vithin  the  last  twenty  years  a  number  of  American 
petrographers  have  made  noteworthy  contributions  to 
the  science  of  rock  classification,  and  with  the  coopera- 
tion of  the  field  geologist  who  has  gradually  become  more 
and  more  painstaking  in  the  matter  of  collecting  and 
labeling  rock  specimens  for  future  study,  they  hope  to 
evolve  from  the  present  classification  which  is  marred 
by  a  complexity  of  nomenclature,  a  logical  and  compre- 
hensive system  of  classification  which  will  approach  in 
construction  as  closely  as  possible  a  truly  natural  arrange- 
ment. 

Among  the  earlier  American  petrographers  who  made 
valuable  contributions  toward  the  development  of  the 
science  are  J.  F.  Kemp,  J.  S.  Diller,  Whitman  Cross,  J.  P. 
Iddings,  and  F.  D.  Adams. 


THE  ELEMENTS  OF  OPTICS  13 


PART  ONE.     OPTICAL  MINERALOGY. 

CHAPTER  1. 

The  Elements  of  Optics,  and  the  Application  of  Polarized 
Light  to  Crystalline  Substances. 

The  Nature  of  Light. — Light  is  a  form  of  energy 
which  in  a  homogeneous  medium  as  the  ether  is  trans- 
mitted in  a  rapid  wave  motion  in  straight  lines  with  no 
change  in  the  direction  of  propagation.  This  wave  mo- 
tion is  considered  to  be  a  resultant  of  simple  harmonic 
motion  and  a  uniform  motion  at  right  angles  to  this.  In 
other  words,  wave  motion  is  a  vibration  which  takes 
place  at  right  angles  to  the  direction  of  propagation  of 
the  light. 

A  ray  of  light  is  a  line  which  designates  the  direction 
of  transmission  of  the  wave.  The  intensity  of  light  de- 
pends upon  the  rate  or  wave-length  of  the  vibrations. 
Color  sensation  is  determined  by  the  number  of  waves 
of  light  which  reach  the  eye  in  a  given  time.  The  wave- 
length for  red  light  is  760  millionths  of  a  millimeter, 
and  the  wave-length  for  violet  light  is  397  millionths  of 
a  millimeter.  White  light  is  the  sum  of  light  of  all  these 
various  wave-lengths.  The  velocity  of  light  of  all  colors 
in  vacuo  is  the  same,  and  is  about  300,000  km  per  second. 

Isotropic  Media. — Light  is  transmitted  with  equal 
velocity  in  all  directions  in  certain  media,  as  air,  water, 
and  glass.  Light  which  is  transmitted  through  such  a 
medium  if  it  finds  its  source  in  that  medium  will  be  propa- 
gated as  spherical  waves,  in  which  the  wave-front  or 


14  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

wave-surface  forms  a  continuous  surface,  and  all  points 
on  that  surface  are  equidistant  from  the  source.  A  ray 
of  light  is  perpendicular  to  its  wave-front. 

In  an  isotropic  substance,  this  wave-surface  may  be 
represented  by  the  surface  of  a  sphere.  Any  plane  pass- 
ing through  this  imaginary  sphere  in  any  position  will 
have  a  circular  outline.  Gases,  liquids,  amorphous  sub- 
stances as  volcanic  glass,  and  crystals  of  the  isometric 
system,  are  isotropic  substances.  The  velocity  of  trans- 
mission of  light  through  these  substances  is  independent 
of  the  direction  of  vibration. 

Anisotropic  Media. — In  anisotropic  media  (as  op- 
posed to  isotropic  media) ,  the  velocity  with  which  light 
is  propagated  varies  with  the  direction.  All  substances 
which  are  not  amorphous  or  which  do  not  belong  to  the 
isometric  system  are  optically  anisotropic. 

Anisotropic  crystals  are  divided  into  uniaxial  and 
biaxial  crystals. 

Uniaxial  Crystals. — In  uniaxial  crystals,  only  one 
direction  exists  in  which  there  is  no  double  refraction  of 
light.  This  is  in  the  direction  of  the  vertical  crystallo- 
graphic  axis,  which  is  called  the  optic  axis.  It  lies  in  the 
direction  of  either  the  greatest  or  least  ease  of  vibration. 
The  wave-front  which  represents  the  optical  structure 
of  uniaxial  crystals  is  an  imaginary  spheroid  of  revolu- 
tion in  which  the  optic  axis  is  the  axis  of  revolution.  A 
plane  passing  through  the  spheroid  in  any  direction  at 
right  angles  to  the  optic  axis  has  a  circular  outline.  Any 
other  section  has  an  elliptical  outline.  Tetragonal  and 
hexagonal  crystals  are  uniaxial. 

Biaxial  Crystals. — In  biaxial  crystals  there  are  two 
directions  corresponding  in  character  to  the  one  optic 
axis  of  uniaxial  crystals,  which  gives  rise  to  the  term 
biaxial.  The  wave-front  which  represents  the  optical 
structure  of  biaxial  crystals  is  an  imaginary  ellipsoid 


THE  ELEMENTS  OF  OPTICS  15 

with  three  unequal  rectangular  axes.  A  plane  passing 
through  this  ellipsoid  in  any  direction  at  right  angles  to 
either  of  the  optic  axes  has  a  circular  outline.  Any  other 
section  has  an  elliptical  outline.  Orthorhombic,  mono- 
clinic  and  triclinic  crystals  are  biaxial. 

Index  of  Refraction. — The  previous'  discussion  has 
been  concerned  with  light  which  has.  passed  through 
homogeneous  media.  If  a  system  of  light  waves  of  the 
same  wave-length  passes  obliquely  from  one  medium  into 
another,  there  will  be  a  change  in  the  direction  of  trans- 
mission depending  upon  the  relative  ease  or  difficulty 
with  which  the  light  may  penetrate  the  new  medium.  If 
the  second  medium,  such  as  glass,  is  optically  more  dense 
than  the  first  medium,  such  as  air,  that  portion  of  the 
wave-front  which  first  strikes  the  glass  will  experience 
a  greater  difficulty  in  transmission,  and  its  velocity  will 
be  reduced,  while  the  remainder  of  the  wave-front  is  still 
traveling  with  the  same  velocity  in  the  air.  When  this 
portion  of  the  wave-front  finally  reaches  the  glass,  it  has 
gained  upon  the  first  portion,  with  a  result  that  the  wave 
will  have  suffered  a  deflection  from  its  original  course. 
From  this  position  the  various  portions  of  the  wave-front 
continue  through  the  glass  with  equal  velocities. 

This  phenomenon  is  called  refraction.  It  is  a  change 
of  direction  at  the  bounding  surface.  Refraction  is 
toward  the  perpendicular  (to  the  bounding  surface) 
when  the  passage  of  a  light  ray  is  from  the  rarer  to 
the  denser  medium,  and  away  from  the  perpendicular 
in  the  opposite  case. 

In  Fig.  1,  D  C  is  the  bounding  surface  between  two 
media,  of  which  the  lower  is  optically  denser  than  the 
upper.  G  H  is  a  perpendicular  to  the  bounding  surface. 
Angle  i  is  the  angle  of  incidence  and  angle  p  is  the  angle 
of  refraction.  A  constant  relation  exists  between  the 
sines  of  these  angles  regardless  of  the  direction  of  trans- 


16  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

mission,  which  may  be  expressed  as  follows:  the  sine 
of  the  angle  of  incidence  bears  a  constant  ratio  to  the 
sine  of  the  angle  of  refraction.  This  ratio  may  be 

sin  i 

expressed  by  the  equation  -  =  n,  in  which  n  is 

sin  r 

the  index  of  refraction  and  is  inversely  proportional  to 

c 

^~~~ 

F 


Fig;.  1.     Reflection  and  single  refraction. 
(Winchell.) 

the  wave  velocity.  In  this  formula  there  are  two  lim- 
iting relations  to  be  considered.  If  i  —  0,  r  =  0,  in 
which  case  the  angle  of  refraction  becomes  zero.  Thus, 
by  perpendicular  incidence,  the  ray  proceeds  in  the 
second  medium  without  any  change  in  direction.  If 

i  =  90, =  n  or  sin  r  = — -.    This  value  of  r  is  known 

sin  r  n 

as  the  critical  angle,  or  angle  of  total  reflection,  and  may 


THE  ELEMENTS  OF  OPTICS  17 

be  defined  as  that  angle  beyond  which  no  light  passes 
from  denser  to  rarer  media.  All  light  may  pass  from 
a  rarer  to  a  denser  medium,  but  the  amount  of  light 
which  may  pass  from  the  denser  to  the  rarer  medium 
is  limited  by  the  critical  angle.  The  critical  angle  is 
a  constant  for  the  substance. 
Thus  for  water  n  =  1.335. 

1 

sin  r  = 

1.335. 

r  =  48°  35'. 
And  for  diamond,  n  =  2.42. 

sin  r  = 

2.42 

r  =  24°  25'. 

If  light  is  to  pass  from  water  into  air,  the  rays  must 
strike  the  surface  at  angles  less  than  48°  35',  whereas 
if  light  is  to  pass  from  diamond  into  air,  the  rays  must 
strike  the  surface  at  angles  less  than  24°  25'.  Evidently 
more  light  can  enter  the  diamond  than  can  directly  es- 
cape, and  this  fact  is  responsible  for  the  brilliancy  of 
the  gem.  The  greater  the  index  of  refraction,  the  smaller 
will  be  the  critical  angle.  In  a  cut  diamond,  the  facets 
are  arranged  so  that  most  of  the  light  is  totally  reflected. 

Most  substances  have  a  value  for  n  ranging  be- 
tween 1  and  2.  The  following  table  gives  the  indices  of 
refraction  for  a  variety  of  substances : 

Ice      .          .          .          .         1.310      Quartz    ....    1.547 

1.601 
.  1.75 

1.814 
.  1.952 

2.369 
.  2.429 

2.712 
.  3.016 


Water     . 

.     1.335 

Calcite 

, 

Alcohol 

1.36 

Methylene  iodide 

Fluorite 

.    1.434 

Garnet 

Common  glass     . 

1.435 

Zircon     . 

Olive  oil 

.    1.47 

Sphalerite 

Canada  balsam  . 

1.536-1.549 

Diamond 

Rock  salt 

.    1.544 

Rutile 

Bromoform 

1.59 

Pyrargyrite 

. 

18  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

An  adamantine  luster  is  characteristic  of  minerals 
with  an  index  of  refraction  above  1.9. 

Ordinarily,  the  index  of  refraction  of  a  substance  is 
determined  by  passing  the  incident  ray  into  the  sub- 
stance from  air,  but  other  media  than  air  might  be  used. 
The  index  of  refraction  of  the  substance  in  air  is  the 
product  of  its  index  in  the  medium  by  the  index  of 
that  medium  in  air.  The  index  of  refraction  of  air 
when  referred  to  a  vacuum  is  1.000294. 

Elementary  phenomena  in  refraction,  such  as  the  ap- 
parent bending  of  a  stick  of  wood  when  partially  sub- 
merged in  water,  were  no  doubt  observed  in  early  times. 
The  constant  ratio  between  sin  i  and  sin  r  was  first 
established  by  Descartes  in  1637,  but  it  was  not  until 
Newton  succeeded  in  producing  a  colored  spectrum  by 
a  prismatic  decomposition  of  white  light  that  the  full 
importance  of  n  was  realized. 

Double  Refraction. — Double  refraction  is  the  prop- 
erty possessed  by  all  anisotropic  crystals  of  resolving 
a  light  ray  into  two  rays  polarized  at  right  angles  to 
each  other  and  traveling  in  different  directions.  This  is 
due  to  the  fact  that  upon  entering  the  anisotropic  me- 
dium the  vibrations  of  light  are  made  to  conform  to 
the  molecular  structure  of  the  medium.  In  other  words, 
light  travels  with  different  velocities  in  different  crys- 
tallographic  directions  in  the  same  substance. 

The  ray  advances  with  the  greatest  velocity  when 
it  is  vibrating  parallel  to  the  direction  of  the  greatest 
ease  of  vibration,  and  with  least  velocity  when  vibrat- 
ing parallel  to  the  direction  of  least  ease  of  vibration. 
These  rays  obviously  have  different  indices  of  refrac- 
tion. The  ray  which  follows  the  usual  laws  of  single 
refraction  is  called  the  ordinary  ray,  expressed  by  0. 
The  other  ray  is  called  the  extraordinary  ray  (E)  be- 


THE  ELEMENTS  OF  OPTICS 


19 


cause  it  does  not  follow  the  usual  laws  of  single  re- 
fraction. 

When  a  ray  of  light  enters  an  anisotropic  medium 
perpendicular  to  its  surface,  the  ordinary  ray  passes 
through  without  suffering  refraction,  provided  the  sur- 


l'"ig.   2.     Plane   wave   advancing:   perpendicular 
to  the  vertical  axis,   showing  ether 
vibration  and  retardation  of 
the   O   and   E   rays. 
(Winchell.) 

face  through  which  it  emerges  is  parallel  to  the  sur- 
face through  which  it  enters.  The  extraordinary  ray 
is  diverted.  To  this  rule  the  following  exceptions  must 
be  noted.  If  the  original  ray  enters  a  substance  per- 
pendicular to  the  surface  and  at  the  same  time  par- 


20 


OPTICAL  MINERALOGY  AND  PETROGRAPHY 


allel  to  an  optic  axis,  there  is  no  refraction  nor  polar- 
ization. If  perpendicular  to  the  surface  and  to  an  optic 
axis,  there  is  no  refraction  but  there  is  a  division  into  two 
rays  traveling  with  different  velocities  and  polarized  at 
right  angles  to  each  other. 


Fig.  S.     Oblique  incidence   on   a  gurface 
parallel  to  the  optic  axis.    (Winchell.) 

When  the  two  rays  emerge  from  the  substance,  they 
resume  parallelism,  but  the  waves  of  one  are  slightly 
in  advance  of  the  waves  of  the  other.  Such  waves  are 
said  to  interfere  with  each  other,  producing  light  of 
different  colors.  Upon  this  phenomenon  is  based  much 


THE  ELEMENTS  OF  OPTICS  21 

of  the  work  done  in  examining  minerals  in  thin  section 
under  the  microscope  in  parallel  polarized  light. 

Interference. — Two  waves  of  like  length  and  ampli- 
tude, if  propagated  in  the  same  direction  and  meeting 
in  the  same  phase,  unite  to  form  a  wave  of  double  am- 
plitude. If  these  waves  differ  in  phase  by  half  a  wave- 
length or  an  odd  multiple  of  this,  they  interfere  in  such 
a  way  as  to  extinguish  each  other.  For  other  relations 
of  phase  falling  between  these  extreme  cases  they  also 
interfere  with  each  other,  forming  a  new  resultant  wave, 
differing  in  amplitude'from  each  of  the  component  waves. 
We  are  assuming  here  the  use  of  monochromatic  light 
waves,  or  light  waves  of  like  length.  If  ordinary  white 
light  is  employed,  the  waves  in  case  of  interference  will 
be  indicated  by  the  appearance  of  the  colors  of  the 
spectrum. 

Polarization. — Ordinary  light  is  propagated  by  trans- 
verse vibrations  of  the  ether  which  take  place  in  all 
directions  about  the  line  of  propagation.  Plane  polar- 
ized light  is  propagated  by  ether  vibrations  which  take 
place  in  one  plane  only.  This  phenomenon  is  called  polar- 
ization. It  may  be  described  as  a  change  in  the  char- 
acter of  reflected  or  transmitted  light,  which  diminishes 
its  power  of  being  further  reflected  or  transmitted. 

Light  is  polarized  by  reflection,  by  single  refraction, 
and  by  double  refraction.  The  plane  of  polarization  of 
light  polarized  by  reflection  is  defined  as  the  plane  con- 
taining the  incident  and  the  reflected  rays,  the  vibra- 
tions taking  place  at  right  angles  to  it.  The  plane  of 
polarization  of  the  refracted  ray  is  the  plane  at  right 
angles  to  the  vibration  direction,  consequently  at  right 
angles  to  the  plane  of  the  incident  and  the  reflected  rays. 
That  light  is  polarized  when  reflected  may  be  shown 
experimentally  by  the  use  of  two  reflecting  surfaces. 

Nicol  Prism. — The  Nicol  prism,  so  named  after  its 


22  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

inventor,  Nicol,  is  a  device  for  producing  polarized  light. 
It  consists  of  a  clear  transparent  crystal  of  calcite  known 
as  Iceland  spar,  as  it  is  obtained  almost  exclusively 
from  caves  in  certain  basalts  in  Iceland.  The  vertical 
faces  are  natural  cleavage  faces,  in  which  the  end  cleav- 
ages, inclined  71  degrees  to  the  obtuse  edges  of  the 
prism,  are  ground  down  and  polished  so  as  to  make 
an  angle  of  68  degrees  with  the  obtuse  vertical  edges. 
It  is  then  cut  diagonally  in  two  parts  perpendicular  to 


fig.   4.     Side   view   of 
the  Nicol   prism. 


Fig.  5.     End  view  of 
the  Nicol   prism. 


the  short  diagonal  of  the  end  face.  The  two  parts  are 
cemented  together  in  their  original  position  by  Canada 
balsam,  a  resin  obtained  from  a  species  of  fir.  It  has 
an  index  of  refraction  of  1.536.  Since  calcite  is  a  doubly 
refracting  substance,  the  Nicol  prism  refracts  a  ray  of 
light  into  two  rays  —  the  ordinary  ray,  having  an  index 
of  1.658,  and  the  extraordinary  ray,  having  an  index 
of  1.486. 

The  angle  at  which  the  two  new  planes  are  polished, 
as  well  as  the  angle  at  which  the  crystal  is  cut,  are 
so  calculated  that  the  ordinary  ray  will  strike  the  bal- 


THE  ELEMENTS  OF  OPTICS  23 

sam  at  an  angle  greater  than  the  critical  angle.  Conse- 
quently, the  ordinary  ray  is  totally  reflected  and  is  ab- 
sorbed in  the  blackened  walls  of  the  cork  mountings. 
The  extraordinary  ray  passes  through  the  balsam  as  a 
completely  polarized  ray  which  is  vibrating  in  a  known 
direction,  namely,  parallel  to  the  short  diagonal  of  the 
calcite  rhomb.  When  two  Nicol  prisms  have  their  short 
diagonals  parallel,  light  passes  through  without  being 
changed  except  for  a  decrease  in  intensity.  If  one  of  the 
nicols  is  revolved,  the  light  gradually  diminishes  until  the 
nicols  are  at  90  degrees  to  each  other,  when  darkness 
results. 


24  OPTICAL  MINERALOGY  AND  PETROGRAPHY 


CHAPTER  2. 
The  Polarizing  Microscope   and   Its   Parts. 

The  Polarizing  Microscope. — In  order  to  ascertain 
the  peculiarities  of  minerals  of  each  of  the  crystallo- 
graphic  systems  as  they  are  manifested  in  polarized 
light,  the  polarizing  microscope  is  used.  This  instru- 
ment is  applicable  to  the  study  of  the  form,  optical  prop- 
erties, and  mutual  relations  of  the  minerals  as  they  are 
found  in  thin  sections  of  rocks,  making  it  a  valuable 
aid  to  geological  research.  It  is  likewise  used  to  great 
advantage  in  the  study  of  small  isolated  crystals,  or 
fragments  of  crystals.  A  determination  of  the  follow- 
ing characteristics  of  the  unknown  mineral  is  of 
particular  value  in  its  identification:  crystal  form  as 
shown  in  outline,  direction  of  cleavage  lines,  refrac- 
tive index,  light  absorption  in  different  directions,  the 
isotropic  or  anisotropic  character,  position  of  the  axial 
plane  and  the  nature  of  the  axial  interference  figures, 
the  strength  and  character  (positive  or  negative)  of 
the  double  refraction,  presence  and  nature  of  inclusions, 
type  of  twinning. 

In  addition  to  the  parts  essential  to  the  ordinary 
microscope  the  polarizing  microscope  contains  the  fol- 
lowing parts:  two  Nicol  prisms,  a  lens  for  convergent 
polarized  light,  a  rotating  stage,  and  an  ocular  with  cross 
hairs. 

Nicol  Prisms. — The  effects  due  to  polarized  light  can- 
not usually  be  distinguished  except  by  a  combination  of 
two  Nicol  prisms.  The  upper  nicol  is  not  revolvable, 


THE  POLARIZING  MICROSCOPE  AND  ITS  PARTS          25 


fi.      The   Fiiesa    microscope. 


26  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

and  is  placed  in  a  support  between  the  ocular  and  the 
objective.  It  can  be  pushed  in  or  out  of  the  tube  at  will. 
It  is  called  the  analyzer. 

The  lower  nicol,  which  is  revolvable,  is  placed  be- 
neath the  stage.  For  ordinary  work,  its  principal  sec- 
tion (i.e.,  its  shorter  diameter)  is  placed  at  right  angles 
with  the  principal  section  of  the  upper  nicol.  It  may  be 
raised  or  lowered  without  disturbing  the  centering.  This 
nicol  is  called  the  polarizer. 

The  principal  section  of  the  analyzer  is  left  and  right, 
and  that  of  the  polarizer  is  front  and  rear.  In  this  posi- 
tion the  field  is  dark  and  the  nicols  are  "crossed." 

When  a  thin  section  is  examined  over  the  lower 
nicol  or  between  two  nicols  without  the  convergent  light, 
it  is  said  to  be  done  in  parallel  polarized  light.  When 
the  lower  nicol  is  used  alone,  its  vibration  plane  must 
be  known.  A  simple  test  is  to  place  a  cleavage  frag- 
ment of  calcite  within  the  field  of  view.  It  has  a  high 
relief  when  its  long  diagonal  is  parallel  to  the  plane 
of  vibration  of  the  nicol. 

A  section  of  biotite  cut  at  right  angles  to  its  cleav- 
age has  its  greatest  absorption  when  its  cleavage  direc- 
tion is  parallel  to  the  plane  of  vibration  of  the  polar- 
izer. Consequently,  it  is  darkest  in  this  position.  Tour- 
maline, on  the  other  hand,  extinguishes  vibrations  at 
right  angles  to  the  optic  axes,  i.e.,  it  absorbs  the  ordi- 
nary ray,  and  only  the  light  rays  vibrating  parallel  to 
the  crystallographic  axis  c  emerge. 

By  removing  the  Nicol  prism  from  the  tube,  the 
separating  plane  of  the  balsam  along  which  the  two 
fragments  of  calcite  were  cut  may  be  seen  upon  looking 
through  the  prism  at  an  angle.  The  vibration  direc- 
tion of  the  ray  which  passes  through  the  prism  (the  ex- 
traordinary ray)  is  normal  to  the  layer  of  balsam. 

The  polarizer  may  be  removed  from  the  microscope 


THE  POLARIZING  MICROSCOPE  AND  ITS  PARTS          27 

and  light  reflected  from  a  horizontal  surface,  such  as 
a  plate  of  glass  or  a  table  top,  examined  through  it.  Since 
light  is  polarized  in  a  plane  parallel  to  the  reflecting 
surface,  the  polarizing  plane  of  the  nicol  lies  at  right 
angles  to  the  reflecting  surface  when  the  latter  appears 
dark. 

Lens  for  Convergent  Light. — When  the  operation  de- 
mands convergent  light,  a  powerful  convergent  lens  can 
be  thrown  into  the  tube  of  the  microscope  over  the 
polarizer  by  means  of  a  lever  beneath  the  stage.  This  lens 
may  be  raised  with  the  lower  nicol  until  the  surface 
of  the  lens  is  practically  in  contact  with  the  glass  slide 
holding  the  thin  section. 

The  Rotating  Stage. — The  stage  is  a  circular  table 
upon  which  the  thin  section  is  placed  for  examination. 
The  edge  has  a  graduated  scale  and  vernier  reading  to 
minutes.  The  center  of  the  stage  must  coincide  with 
the  optical  center  of  the  tube.  Centering  is  done  by 
means  of  two  centering  screws,  90  degrees  apart,  lo- 
cated on  the  lower  end  of  the  tube.  The  thin  section  is 
held  in  place  by  two  spring  object  clips.  Recent  forms 
of  microscopes  are  equipped  with  mechanical  stages 
which  have  freedom  of  movement  in  a  left-and-right 
direction  and  in  a  front-and-rear  direction,  thus  allowing 
a  rapid  inspection  of  every  part  of  the  section. 

Gross  Hair.-^Cross  hairs  are  placed  in  the  ocular 
at  right  angles  to  each  other,  one  running  left  and  right 
and  the  other  front  and  rear,  in  agreement  with  the 
principal  sections  of  the  nicols  when  crossed. 

To  measure  a  plane  angle  in  thin  section,  or  the  in- 
terfacial  angle  of  a  small,  flat  crystal,  the  stage  is  cen- 
tered with  the  intersection  of  the  two  edges  at  the  cen- 
ter of  the  cross  hairs.  A  reading  is  made  when  one 
edge  of  the  crystal  is  parallel  to  the  left-and-right  cross 
hair,  then  the  stage  is  revolved  until  the  other  edge  is 


28  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

parallel  to  the  same  cross  hair  but  on  the  opposite  side 
of  the  center.  Another  reading  is  taken.  The  difference 
between  the  two  readings  is  the  external  angle. 

Other  parts  of  the  microscope  which  deserve  expla- 
nation and  suggestions  as  to  proper  use  are  taken  up  in 
order. 

The  Mirror. — The  mirror  which  is  attached  to  the 
substage  reflects  light  from  the  source  to  the  object. 
A  plane  mirror  forms  one  side  and  a  concave  mirror 
the  other.  The  former  is  used  for  low  magnification, 
where  a  weak  light  is  sufficient.  The  latter  is  used  for 
higher  magnifications.  This  mirror  concentrates  the 
light  by  converging  the  rays  included  within  an  angular 
aperture  of  about  40  degrees.  For  still  higher  mag- 
nifications and  for  all  phenomena  observed  in  convergent 
light  the  condensing  lens  is  used. 

A  proper  use  of  the  mirror  is  essential  to  the  most 
efficient  use  of  the  microscope.  When  parallel  rays,  such 
as  ordinary  daylight,  are  used,  they  are  reflected  from 
the  mirror  with  a  slight  loss  of  intensity.  They  are  re- 
flected from  the  concave  mirror  with  increased  intensity, 
the  rays  coming  together  at  the  focal  point.  If  the 
source  of  light  is  close  to  the  instrument,  the  focal  length 
is  larger.  To  meet  this  adjustment,  the  mirror  is  at- 
tached to  a  sliding  vertical  bar.  Since  the  condensing  lens 
has  its  focus  some  distance  above  its  upper  surface,  the 
plane  mirror  is  used  in  connection  with  it. 

The  Objective. — Objectives  are  classified  according  to 
their  magnification.  An  objective  of  low  power  has  a 
focal  length  above  13  mm  and  a  magnification  less  than 
15  diameters;  it  is  of  medium  power  when  its  focal 
length  is  between  12  and  5  mm  and  its  magnification 
is  40  diameters;  of  high  power  when  its  focal  length 
is  less  than  4.5  mm  and  its  magnification  exceeds  40 
diameters.  The  objectives  most  commonly  used  are 


THE  POLARIZING  MICROSCOPE  AND  ITS  PARTS          29 

numbers  3  and  7,  the  former  for  searching  out  an  object 
and  for  making  the  preliminary  examination,  and  the 
latter  for  convergent  light  and  high  power. 

A  thin  section  may  be  considered  as  made  up  of  a 
series  of  planes  superimposed  one  above  the  other,  only 
one  of  which  may  be  seen  for  one  adjustment  of  focus. 
With  low-power  objectives  one  can  see  objects  lying  in 
slightly  different  planes,  but  with  high-power  lenses  this 
is  impossible,  as  the  depth  of  focus  diminishes  inversely 
as  the  numerical  aperture.  The  brightness  of  the  image 
increases  as  the  square  of  the  numerical  aperture. 

Resolving  Power. — The  resolving  power  of  an  ob- 
jective is  that  property  by  virtue  of  which  one  is  able 
to  see  the  finer  details  of  an  object.  This  resolving  power 
increases  with  the  number  and  obliquity  of  the  rays 
coming  from  the  object,  consequently  an  immersion  fluid 
by  increasing  the  number  of  rays  brought  to  the  object 
increases  the  resolving  power.  In  petrographic  work 
no  very  great  magnifying  powers  are  required,  and  im- 
mersion lenses  are  not  much  used  except  for  particular 
kinds  of  work. 

When  two  points  are  removed  from  the  eye  6,876 
times  the  distance  separating  them  they  will  appear 
as  a  single  point.  The  eye  is  able  to  distinguish  only 
about  250  lines  to  an  inch.  Thus  pleurosigma  angulatum 
with  about  50,000  lines  to  the  inch  can  be  resolved  by  a 
one-half  inch  objective  so  as  to  be  clearly  seen  with  a 
three-quarters  inch  ocular  but  not  with  one-and-one-half 
inch.  A  much  smaller  line  may  be  seen  than  the  inter- 
val between  two  lines. 

Cost  of  Objectives. — Objectives  with  a  focal  length 
of  25  mm  and  over  cost  about  $4  each;  between  25  and 
10  mm,  $5.50  to  $10;  10  to  3 'mm,  $7  to  $15;  3  to  2  mm, 
about  $20.  Students  are  urged  to  treat  them  with  care. 


30  OPTICAL  MINER*u<)GY  AND  PETROGRAPHY 

Bertrand  "Lens. — In  the  center  of  the  microscope  tube 
above  the  analyzer  is  the  Bertrand  lens,  which  may  be 
thrown  in  or  out  of  the  tube  by  means  of  a  sliding  car- 
rier. It  acts  as  a  small  microscope  which  is  used  with 
the  ocular  to  magnify  interference  figures. 

The  Ocular. — The  Huygens  ocular  which  is  most  gen- 
erally used  in  petrographic  microscopes  consists  of  two 
simple  plane-convex  lenses  placed  with  their  plane  sur- 
faces toward  the  eye.  The  upper  lens  is  known  as  the 
eye  lens  and  the  lower  as  the  collective  or  field  lens.  The 
focal  length  of  the  eye  lens  is  one  third  of  the  field 
lens,  and  they  are  separated  a  distance  equal  to  the  sum 
of  their  focal  lengths.  The  rays  of  light  emerging  from 
the  eye  lens  are  parallel  and  thus  cause  the  eye  less 
fatigue. 

The  cross  hairs  which  are  placed  in  the  eyepiece  are 
made  of  spider  web,  -the  dark  thread  from  the  inside 
of  the  nest  being  the  best. 

Micrometer. — It  is  desirable  at  times  to  measure 
small  distances  such  as  the  dimensions  of  small  crystals. 
A  special  eyepiece  called  the  micrometer  has  been  devised 
•for  this  purpose.  It  contains  a  scale  etched  on  glass.  On 
the  stage  of  the  microscope  a  scale  reading  to  hundredths 
of  a  mm  is  placed.  It  is  then  necessary  to  find  to  how 
many  hundredths  of  a  mm  each  division  of  the  eyepiece 
is  equivalent. 

It  may  be  well  for  the  student,  in  order  to  become 
familiar  with  the  use  of  the  micrometer,  to  construct 
a  table  showing  the  value  of  the  ocular  micrometer  for 
each  objective.  The  stage  micrometer  is  used  for  this 
purpose. 

Adjustment  Screws. — The  tube  carrying  the  eyepiece 
and  objective  has  a  fine  adjustment  screw,  the  edge  of 
which  is  graduated.  It  moVes  against  a  fixed  index  at- 
tached to  the  tube,  by  which  means  the  distance  through 


THE  POLARIZING  MICROSCOPE  AND  ITS  PARTS  31 

which  the  tube  is  raised  or  lowered  can  be  measured 
to  .001  mm. 

The  student  is  advised  as  a  laboratory  illustration 
to  determine  the  amount  which  one  revolution  of  the 
fine  adjustment  screw  raises  the  objective.  To  do  this, 
measure  the  thickness  of  a  glass  plate  or  cover  glass  by 
focusing  carefully  on  the  lower  surface  of  the  glass  and 
then  upon  the  upper  surface.  This  distance  is  meas- 
ured by  the  micrometer  by  setting  the  glass  plate  on  edge, 
slightly  embedded  in  paraffin  or  wax,  or  supported  other- 
wise. 

Use  of  the  Microscope. — The  best  light  for  micro- 
scopic work  is  that  coming  from  the  north;  the  next 
best  from  the  east.  Direct  sunlight  should  never  be  used. 
The  table  should  be  firm,  and  of  a  height  to  suit  the 
convenience  of  the  individual.  The  instrument  should 
be  placed  directly  in  front  of  the  observer,  so  that  both 
hands  can  be  used  for  manipulation. 

The  eye  which  is  not  used  for  observation  should  also 
be  kept  open.  Although  it  may  seem  difficult  at  first 
to  concentrate  the  gaze  on  the  thin  section,  it  will  be 
found  to  be  far  less  fatiguing.  When  using  high  powers, 
the  eye  must  be  kept  very  close  to  the  ocular,  with  low 
powers  slightly  farther  removed.  When  both  nicols  are 
being  used,  more  light  is  advantageous  than  when  only 
one  is  in  use.  As  much  of  the  examination  of  a  thin  sec- 
tion as  possible  should  be  done  with  the  low  powers  in 
order  to  save  a  strain  on  the  eyes. 

The  student  is  particularly  cautioned  whenever  fo- 
cusing with  high  powers  to  focus  upward  and  never 
downward.  If  this  rule  is  followed,  no  thin  sections  will 
be  broken.  Place  the  eye  on  a  level  with  the  stage, 
and  lower  the  tube  slowly  until  the  objective  is  almost 
in  contact  with  the  thin  section.  Then  looking  through 
the  tube,  raise  the  objective  slowly  until  the  portion  of 


32  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

the  section  desired  for  examination  is  in  focus.  If  col- 
orless minerals  such  as  quartz  are  being  examined,  it 
is  well  to  reduce  the  amount  of  the  illumination  and 
look  for  bubbles  or  other  inclusions. 

Proper  care  of  the  nicols  and  lenses  prolongs  their 
life  and  increases  their  efficiency.  They  should  not  be 
exposed  to  severe  sunlight  nor  to  the  heat  from  a  steam 
radiator,  lest  the  cement  soften.  The  lenses  should  be 
kept  free  from  dust.  The  objective  should  never  be 
allowed  to  come  in  contact  with  the  cover  glass. 


METHODS  OF   MINERAL  DETERMINATION  33 


CHAPTER  3. 
General  Methods  of  Mineral  Determination. 

The  determination  of  unknown  minerals  in  thin  sec- 
tion may  be  accomplished  by  the  use  of  one  or  all  of 
the  following  eight  general  operations : 

1.  Determination  of  the  general  physical  properties 
of  minerals  by  ordinary  light. 

2.  Determination  of  the  relative  refractive  index. 

3.  Determination  of  the  relative  double  refraction  or 
birefringence. 

4.  Determination  of  the  axial  interference  figures. 

5.  Determination  of  the  dispersion  of  the  optic  axes. 

6.  Determination  of  the    optical    character    or    the 
character  of  the  double  refraction. 

7.  Determination  of  the  extinction  angle  or  the  rela- 
tion of  the  crystallographic  axes  to  the  axes  of  ether 
elasticity. 

8.  Determination  of  the  presence  or  absence  of  pleo- 
chroism. 

General  Operation  No.  1:  Determination  of  the  Gen- 
eral Physical  Properties  of  Minerals  by  Means  of  Ordi- 
nary Light. 

The  physical  properties  of  minerals  referred  to  in 
this  paragraph  are:  crystal  form,  cleavage,  parting, 
twinning,  and  color.  Ordinary  light  is  light  which  has 
not  been  polarized  to  obtain  which  both  nicols  should  be 
removed  from  the  microscope.  If  the  lower  nicol  is 
difficult  to  remove,  the  observations  are  made  in  plane 


34  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

polarized  light,  which  generally  causes  no  great  difference 
in  the  appearance  of  the  mineral.  The  intensity  of  the 
unpolarized  light,  however,  is  much  greater  than  that  of 
the  polarized. 

Minerals  examined  by  ordinary  light  are  of  two 
classes:  transparent  and  opaque.  The  former  class  is 
examined  by  transmitted  light  for  crystal  form,  cleav- 
age, and  color ;  the  latter  class  by  incident  light  for  crys- 
tal form,  color,  luster,  etc. 

CRYSTAL  FORM.  The  determination  of  crystal  form 
is  not  of  great  importance  in  the  study  of  rock  sections 
for  the  reason  that  individual  crystals  have  not  had  the 
opportunity  for  undisturbed  development,  but  have  been 
hampered  in  their  growth  by  interference  with  neigh- 
boring crystals.  In  certain  porphyries  a  study  of  the 
form  of  the  phenocrysts  often  leads  to  their  identification. 

CLEAVAGE.  Pronounced  cleavage  lines  are  developed 
in  certain  minerals  in  characteristic  directions  during 
the  process  of  grinding  to  thin  section.  The  direction 
and  perfection  of  the  cleavage  cracks  is  indicative.  A 
mineral  possessing  no  cleavage  will  have  irregular 
cracks,  as  quartz. 

Perfect  cleavage  is  a  cleavage  in  which  the  lines  are 
sharp  and  extend  for  considerable  distances.  Examples : 
mica,  fluorite. 

Good  or  distinct  cleavage  is  cleavage  in  which  the 
cracks  are  interrupted  with  offsets,  etc.  Examples: 
augite,  hornblende,  orthoclase. 

Poor  or  indistinct  cleavage  is  very  irregular,  with 
uneven  cracks,  though  they  follow  roughly  certain  direc- 
tions. 

Pinacoidal  cleavage,  as  shown  in  mica,  is  well  devel- 
oped in  one  direction  only.  Prismatic  cleavage,  as 


METHODS  OF   MINERAL  DETERMINATION  35 

shown  by  augite  and  hornblende,  usually  develops  in 
two  planes.  In  certain  minerals  of  the  isometric  and 
hexagonal  systems,  such  as  galena  and  calcite,  three 
good  cleavages  develop. 

Cleavage  angles,  of  course,  depend  upon  the  orien- 
tation of  the  random  section  shown  in  the  thin  section. 
Where  the  section  is  cut  at  right  angles  to  the  cleavage 
planes,  the  angles  are  characteristic.  Hence,  if  one  is 
using  cleavage  fragments,  he  can  orient  it  at  will,  since 
the  flat  faces  will  bear  definite  relations  to  the  crystal- 
lographic  axes. 

PARTING.  Parting  is  a  fracture  often  developed  par- 
allel to  a  certain  cleavage  direction  occurring  along 
planes  of  weakness  as  may  result  from  shearing  or  glid- 
ing planes. 

TWINNING.  Twinning  is  important  in  certain  min- 
erals and  will  be  discussed  in  Chapters  5,  6,  and  7. 

COLOR.  All  colored  minerals  may  be  divided  into 
two  classes :  idiochromatic  and  allochromatic.  Idiochro- 
matic  minerals  are  those  in  which  the  color  is  due  to 
a  property  of  the  mineral  itself,  namely,  its  ability  to 
absorb  light  of  certain  wave-lengths,  although  the  prop- 
erty of  absorption  may  not  be  the  same  in  every  direc- 
tion. Allochromatic  minerals  are  those  in  which  the 
color  is  due  to  inclusions,  which  may  or  may  not  be  dis- 
tinguished under  the  microscope.  The  pigment  may 
be  either  organic  or  inorganic.  Carbon,  nitrogen  and 
hydrogen  have  been  found  in  zircon,  smoky  quartz,  ame- 
thyst, fluorite,  apatite,  calcite,  microcline,  barite, 
halite  and  topaz.  Free  fluorine  has  been  found  in  fluor- 
ite. Traces  of  iron  are  found  in  brown  zircon.  The 
pigment  may  be  thickly  and  evenly  distributed  or  irreg- 
ularly and  so  sparingly  distributed  that  a  thin  section 
appears  colorless. 


36  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

General  Operation  No.  2:  Determination  of  the  Rel- 
ative Refractive  Index. 

Refraction  is  the  change  which  light  undergoes  in 
direction  in  passing  between  two  media  which  differ  in 
density.  The  index  of  refraction  described  under  optics 
may  often  be  judged  by  the  appearance  of  the  mineral 
in  the  liquid  in  which  it  is  mounted,  usually  Canada  bal- 
sam. This  makes  a  convenient  standard  with  which  to 
compare  the  index  of  refraction  of  the  unknown  mineral. 
Although  its  index  varies  slightly  during  the  process 
of  mounting  and  with  age,  only  those  few  minerals 
whose  indices  fall  within  the  limits  of  variation  of  the 
balsam  are  affected.  Balsam  may  retain  its  sticky  con- 
sistency and  low  index  for  forty  years  if  protected  by 
a  cover  glass. 

If  the  mineral  under  examination  and  the  balsam 
have  practically  the  same  index  of  refraction,  the  min- 
eral will  appear  smooth  and  will  be  visible  with  difficulty. 
It  is  then  said  to  have  "low  relief."  If  the  two  have 
quite  different  indices,  the  surface  of  the  mineral  will 
appear  rough  and  the  borders  dark.  Such  a  surface, 
because  of  its  resemblance  to  shagreen,  is  called  a  sha- 
green surface.  The  mineral  is  said  to  have  "high  re- 
lief." This  apparently  rough  surface  is  due  to  inequali- 
ties of  the  surface,  each  elevation  and  depression  re- 
flecting and  refracting  the  light  at  a  different  angle. 
This  irregular  illumination  causes  the  mineral  to  appear 
darker  in  some  spots  and  lighter  in  others.  A  mineral 
embedded  in  Canada  balsam  will  have  high  relief  whether 
its  index  is  lower  or  higher  than  the  liquid. 

In  a  rock  section  where  a  number  of  different  min- 
erals having  different  indices  of  refraction  lie  in  con- 
tact, certain  minerals  appear  to  stand  out  above  the  oth- 
ers in  relief.  Minerals  with  high  indices  seem  to  be 
elevated  from  the  plane  of  the  section.  This  is  because 


METHODS  OF    MINERAL  DETERMINATION  37 

the  rays  of  light  from  the  lower  surface  of  different 
minerals  appear  to  come  from  the  points  of  intersection 
of  the  refracted  rays. 

Since  the  index  of  refraction  of  a  mineral  is  one  of 
its  most  important  optical  properties,  many  methods 
have  been  devised  for  its  identification. 

THE  METHOD  OF  Due  DE  CHAULNES.  By  this  method 
one  may  determine  the  index  of  the  mineral  directly 
by  focusing  a  medium-  or  high-power  objective  accu- 
rately upon  an  object,  and  then  inserting  between  it  and 
the  objective  a  transparent  plate  with  parallel  sides.  The 
image  becomes  blurred.  The  tube  of  the  microscope  is 
raised  until  the  image  is  again  in  focus.  The  amount 
of  change  necessary  is  dependent  upon  the  index  of 
refraction  of  the  plate  and  upon  its  thickness. 

As  a  laboratory  illustration  the  student  is  advised  to 
determine  the  index  of  refraction  of  a  plate  glass  or 
cover  glass  by  this  method.  If  the  student  uses  a  glass 
whose  true  thickness  has  already  been  determined,  the 
index  may  be  obtained  by  measuring  the  apparent  thick- 
ness and  dividing  the  true  thickness  by  this  latter 
amount.  If  the  thickness  of  the  glass  is  not  known,  it 
is  possible  to  determine  the  approximate  true  thickness 
by  focusing  on  a  point  or  scratch  on  another  plate  of 
glass  which  is  to  be  used  for  a  support.  Then  place 
upon  it  the  glass  plate  whose  thickness  is  to  be  deter- 
mined, and  focus  on  its  upper  surface.  This  distance 
is  the  thickness  of  the  glass  plate  plus  the  thickness 
of  the  air  film  separating  the  plate  from  the  support, 
and  should  be  used  only  in  case  the  thickness  of  the 
plate  is  so  great  that  the  thickness  of  the  air  film  be- 
comes negligible.  The  apparent  thickness  can  now  be 
measured  and  the  index  calculated  as  mentioned  above. 

A  correction  for  the  air  film  can  be  made  very  easily 
and  should  always  be  done  if  the  glass  plate  is  thin. 


38  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

Focus  on  the  upper  surface  of  the  glass  plate  and  then 
on  the  lower  surface.  This  distance  through  which  the 
objective  moves  is  the  apparent  thickness  of  the  plate. 
Now  focus  on  the  surface  of  the  support.  This  gives 
the  true  thickness  of  the  plate  and  the  air  film.  Next 
focus  on  the  lower  surface  of  the  plate  and  on  the  upper 
surface  of  the  support.  This  gives  the  true  thickness  of 
the  air  film,  which  can  readily  be  subtracted  from  the 
thickness  of  the  glass  plate  and  air  film.  The  difference 
is  the  true  thickness  of  the  glass  plate.  The  index  can 
now  be  determined  by  dividing  the  true  thickness  by 
the  apparent  thickness  of  the  plate. 

IMMERSION  METHOD.  If  a  drop  of  a  liquid  with  an 
index  equal  to  that  of  the  mineral  is  placed  upon  a  thin 
section  of  the  mineral  without  a  cover  glass,  the  appear- 
ance of  roughness  which  characterizes  the  mineral  in 
air  disappears,  since  there  is  neither  reflection  nor  re- 
fraction at  the  contact,  and  the  light  passes  through 
without  deflection.  If  the  mineral  is  colorless,  it  prac- 
tically disappears  from  view.  By  the  use  of  a  series 
of  immersion  liquids  whose  indices  of  refraction  are 
known,  it  is  possible  to  experiment  with  the  unknown 
mineral  until  a  liquid  is  found  whose  index  of  refraction 
by  the  above  test  corresponds  with  the  index  of  refrac- 
tion of  the  mineral. 

BECKE  METHOD.  By  the  Becke  method,  which  in- 
volves the  use  of  total  reflection  in  connection  with  re- 
fraction, one  may  determine  the  relation  which  the  re- 
fractive index  of  the  unknown  mineral  has  to  that  of 
one  which  is  known  and  which  is  in  contact  with  it. 
Bring  the  focus  directly  upon  the  line  of  separation  of 
the  two  minerals,  using  a  high-power  objective  in  con- 
vergent light.  If  the  condenser  is  lowered  and  the  an- 
alyzing nicol  is  removed,  it  is  observed  that  the  field 
becomes  slightly  darker,  and  a  fine  line  of  white  light 


METHODS  OF   MINERAL  DETERMINATION  39 

sharply  marks  the  contact  of  the  two  minerals.  Upon 
raising  the  objective  very  slightly,  this  thin  line  of  white 
light  will  be  seen  to  shift  from  the  line  of  contact  of 
the  two  minerals  toward  the  mineral  having  the  higher 
index. 

This  phenomenon  is  explained  as  follows:  The  rays 
of  light  which  enter  the  minerals  perpendicular  to  their 
surfaces  undergo  no  refraction  but  pass  directly  through. 
Those  rays  which  enter  the  mineral  having  the  lower 
index  of  refraction  reach  the  plane  of  contact  of  the 
two  minerals  and  are  all  bent  toward  a  normal  to  .this 
plane  passing  through  the  mineral  having  the  higher 
index,  because  they  are  passing  from  a  rarer  to  a  denser 
medium.  The  rays  which  enter  the  mineral  having  the 
higher  index  must  pass  from  a  denser  medium  to  a 
rarer.  In  such  a  case  it  is  remembered  that  all  of  those 
rays  which  strike  the  mineral  of  lower  index  at  an  angle 
greater  than  the  critical  angle  of  the  denser  mineral, 
are  totally  reflected  and  emerge  from  the  upper  surface 
of  the  denser  mineral.  Only  those  rays  pass  into  the 
mineral  of  lower  index  which  strike  the  plane  of  contact 
of  the  two  minerals  at  an  angle  less  than  the  critical 
angle  of  the  mineral  of  higher  index. 

Upon  lowering  the  objective  slightly,  the  white  line 
shifts  toward  the  mineral  with  the  lower  index. 

It  is  advisable  to  use  the  Becke  test  on  contacts  which 
are  nearly  or  quite  vertical.  One  can  easily  determine 
a  vertical  contact  by  shifting  the  focus  and  seeing  that 
the  boundary  remains  sharp  at  all  foci,  and  in  the  same 
position.  The  verticality  of  the  contact  makes  no  differ- 
ence with  the  result,  provided  the  medium  having  the 
lower  index  lies  above.  If  it  lies  below  and  the  incli- 
nation is  great  enough,  the  bright  line  may  appear  to 
move  the  wrong  way. 

According  to  this  method,   differences   of  .001   be- 


40  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

tween  indices  are  noticeable.  It  is  especially  useful  in 
determining  minerals  with  low  indices,  as  sodalite,  leu- 
cite,  or  in  distinguishing  between  orthoclase  and  quartz. 
Since  the  mean  refractive  index  of  quartz  is  about  the 
same  as  andesine,  is  higher  than  orthoclase,  albite  and 
oligoclase,  and  less  than  labradorite,  bytownite  and  anor- 
thite,  certain  definite  inferences  may  be  drawn  regard- 
ing these  minerals  in  contact. 

The  following  scale  of  refringence  (after  Winchell) 
will  aid  the  student  to  estimate  the  value  of  the  mean 
index  of  refraction  of  minerals  in  thin  section  by  means 
of  "reliefs." 

SCALE  OF  REFRINGENCE. 

Very  low  refringence.  Example,  fluorite,  n  =  1.434. 

Low  refringence.  Example,  quartz,  n  =  1.547. 

Moderate  refringence.  Example,  hornblende,  n  =  1.642. 

High  refringence.  Example,  augite,  n  =  1.715. 

Very  high  refringence.  Example,  zircon,  n  =  1.952. 

The  "negative"  relief  seen  in  fluorite  is  caused  by  the 
total  reflection  of  light  striking  the  lower  surface  of 
the  mineral.  . 

The  indices  of  an  unknown  mineral  compared  with 
those  of  any  known  mineral  by  the  Becke  method  will 
always  give  at  least  one  limit,  which  in  connection  with 
the  visible  amount  of  relief  may  be  sufficient. 

General  Operation  No.  3:  Determination  of  the  Rel- 
ative Double  Refraction  or  Birefringence. 

ISOTROPIC  CRYSTALLINE  SUBSTANCES.  —  ISOMETRIC 
MINERALS.  Between  crossed  nicols,  isometric  minerals 
remain  dark  in  thin  section  during  the  entire  revolution 
of  the  stage.  Such  minerals  allow  the  rays  to  vibrate 
with  equal  ease  in  all  directions  regardless  of  the  direc- 
tion in  which  the  section  is  cut.  They  have  no  inter- 


METHODS  OF   MINERAL  DETERMINATION  41 

ference  colors  in  parallel  polarized  light  nor  interference 
figures  in  convergent  light. 

This  is  due  to  the  fact  that,  in  isotropic  substances, 
light  is  transmitted  with  equal  velocities  in  all  directions ; 
hence  the  velocity  of  light  transmission  is  independent 
of  the  direction  of  vibration. 

Certain  important  isometric  minerals,  as  pyrite  and 
magnetite,  are  opaque.  These  minerals  are  examined 
and  identified  by  other  means  than  by  polarized  light. 

ANISOTROPIC  SUBSTANCES.  Uniaxial  minerals,  or 
minerals  of  the  tetragonal  and  hexagonal  systems. 

A  ray  of  light  emerging  from  the  polarizer  (lower 
nicol)  is  vibrating  in  one  plane  only,  left  and  right. 
Upon  entering  a  thin  section  of  a  tetragonal  or  hexag- 
onal mineral  cut  perpendicular  to  the  vertical  crystallo- 
graphic  axis  c,  the  light  is  not  disturbed,  because  about 
this  axis  vibrations  take  place  with  equal  ease  in  all  di- 
rections perpendicular  to  it.  In  this  one  direction,  light 
is  singly  refracting. 

The  optic  axis  is  that  direction  in  a  doubly  refracting 
substance  in  which  light  is  singly  refracting.  Hence 
in  uniaxial  minerals  the  crystallographic  axis  c  coincides 
with  the  optic  axis. 

This  ray  of  light  emerging  from  the  mineral  is  in- 
tercepted by  the  analyzer  so  as  to  produce  darkness.  The 
student  should  take  the  precaution,  upon  observing  a 
mineral  which  remains  dark  throughout  a  complete  rev- 
olution of  the  stage,  to  determine  whether  it  is  an  iso- 
metric mineral  or  an  isotropic  mineral  cut  perpendicu- 
lar to  an  optic  axis. 

A  ray  of  light  from  the  polarizer  entering  the  thin 
section  of  an  anisotropic  mineral  in  any  other  direc- 
tion than  perpendicular  to  the  optic  axis  is  doubly  re- 
fracted and  polarized,  the  extraordinary  and  the  ordi- 
nary rays  advancing  in  different  directions,  the  former 


42  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

taking  the  oblique  direction.  These  rays  are,  of  course, 
vibrating  perpendicular  to  each  other,  and  perpendicular 
to  their  direction  of  propagation.  The  extraordinary 
ray  vibrates  in  a  plane  containing  the  incident  ray  and 
optic  axis.  This  plane  is  called  the  principal  optic  section. 

On  emerging  from  the  thin  section,  the  extraordinary 
ray  is  more  or  less  advanced  than  the  ordinary  ray. 
Upon  reaching  the  analyzer,  each  of  these  rays  is  again 
resolved  into  two  rays  —  an  extraordinary  and  an  ordi- 
nary—  the  two  extraordinary  rays  vibrating  in  one 
plane,  and  the  two  ordinary  rays  in  a  plane  at  right 
angles.  Upon  reaching  the  layer  of  Canada  balsam,  the 
ordinary  rays  are  totally  reflected  and  absorbed.  The 
two  extraordinary  rays  emerge  from  the  analyzer  in  a 
uniform  direction,  but  not  equally  advanced,  conse- 
quently in  different  phases.  This  interference  produces 
color.  If  the  rays  have  a  difference  of  phase  of  one  half 
of  a  wave-length,  or  any  uneven  multiple  thereof,  dark- 
ness will  result. 

COLOR.    The  kind  of  color  produced  depends  upon : 

1.  The  mineral. 

2.  The  thickness  of  the  section. 

3.  The  direction  in  which  the  section  is  cut. 

The  amount  of  color  depends  upon  the  angle  between 
the  principal  optic  section  and  the  principal  section  of 
either  nicol.  The  color  is  least  when  the  angle  is  0  de- 
grees, and  greatest  when  the  angle  is  45  degrees,  each 
of  these  conditions  occurring  four  times  in  one  revolu- 
tion of  the  section.  Upon  this  phenomenon  is  based 
the  determination  of  the  angle  of  extinction  (General  Op- 
eration No.  6). 

AXES  OF  ETHER  VIBRATION.  The  direction  in  which 
ether  vibrates  in  anisotropic  minerals  with  the  greatest 
ease  is  called  the  greatest  axis  of  ether  vibration.  It 


METHODS  OF    MINERAL  DETERMINATION  43 


Fig.  7.     Changes  of  light  in  passing  through 
a  petrographical   microscope. 


44  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

is  denoted  by  the  letter  X.  The  index  of  refraction  of 
light  vibrating  in  this  direction  is  expressed  by  Wp. 

The  direction  in  which  ether  vibrates  in  anisotropic 
minerals  with  the  least  ease  is  called  the  least  axis  of 
ether  vibration.  It  is  denoted  by  the  letter  Z.  The  in- 
dex of  refraction  of  light  vibrating  in  this  direction  is 
expressed  by  ng. 

In  uniaxial  minerals,  one  of  these  axes  always  coin- 
cides with  the  vertical  crystallographic  axis  c.  The 
other  axis  is  in  all  directions  at  right  angles  to  this. 
Either  the  greater  or  the  lesser  axis  of  ether  vibration 
may  coincide  with  the  vertical  crystallographic  axis. 

The  value  of  the  maximum  double  refraction  or  bire- 
fringence is  the  difference  between  Hp  and  ng.  Thus, 
for  calcite,  ng  is  1.658  and  n?  is  1.486 ;  ns  —  n»  =  0.172, 
which  indicates  a  very  strong  birefringence.  For  quartz, 
ng  is  1.553  and  np  is  1.544 ;  ns  —  np  =  0.009,  which  indi- 
cates weak  birefringence,  as  the  retardation  of  one  ray 
over  the  other  in  emerging  is  very  slight. 

It  will  be  observed  that  in  certain  minerals  the  in- 
dex of  refraction  of  the  extraordinary  ray  is  greater 
than  that  of  the  ordinary  ray,  and  in  other  minerals 
the  reverse  is  true.  A  further  discussion  of  this  fact 
will  be  taken  up  under  General  Operation  No.  5. 

NEWTON'S  COLOR  SCALE.  Thin  sections  of  anisotropic 
minerals  cut  not  perpendicular  to  an  optic  axis  show 
polarization  colors  between  crossed  nicols.  A  careful 
study  of  these  colors  is  most  important  for  a  successful 
determination  of  unknown  minerals. 

The  color  scale  of  Newton  has  been  adopted  as  a 
standard.  It  consists  of  a  succession  of  interference 
tints  shading  into  each  other.  These  same  tints  are 
produced  by  anisotropic  minerals  in  thin  section.  About 
forty  distinguishable  tints  in  natural  light  have  been 


METHODS  OF    MINERAL  DETERMINATION 


45 


named.     The  colors  are  best  exhibited  by  thin  sections 
which  have  a  thickness  ranging  between  0.01  and  0.06. 

Newton's  color  scale  as  applied  to  the  principal  rock- 
making  minerals  in  sections  0.03  mm  thick  is  given  here- 
with. 

NEWTON'S  COLOR  SCALE. 


Millionth* 
of  a 

Interference  of  Colors 

n-n 

Rock-forming 

mm   Re- 

Between   X   Nicols. 

K       1> 

Minerals. 

tardation 

0 

Black 

30 

Iron  gray. 

0.001 

Leucite. 

60 

Lavender  gray. 

0.002 

Vesuvianite. 

117 

Lavender  gray. 

0.004 

Apatite. 

140 

Grayish  blue. 

0.005 

Beryl. 

153 

Grayish  blue. 

0.005 

Nephelite,  riebeckite. 

180 

Lighter  gray. 

0.006 

Stilbite,  zoisite. 

218 

Lighter  gray. 

0.007 

Orthoclase,  microcline, 

kaolin. 

234 

Greenish  white. 

0.008 

Oligoclase,  albite,  labra- 

dorite. 

250 

White. 

0.009 

Corundum. 

267 

Yellowish  white. 

0.009 

Gypsum,  enstatite. 

281 

Straw  yellow. 

0.009 

Quartz,  sapphire. 

306 

Light  yellow. 

0.010 

Topaz,  rhodonite,  stauro- 

lite. 

332 

Bright  yellow. 

0.011 

Clinochlore,  barite. 

Andalusite. 

390 

Orange  yellow. 

0.013 

Anorthite,  hypersthene. 

433 

Orange  yellow. 

0.014 

Wollastonite. 

474 

Orange  red. 

0.016 

Cyanite. 

575 

Violet  (Sensitive  tint 

No.  1)—  Green,  Yel. 

0.019 

Hedenbergite. 

589 

Indigo. 

0.020 

Tourmaline. 

629 

Blue. 

0.021 

Wernerite. 

667 

Sky  blue. 

0.022 

Augite. 

688 

Sky  blue. 

0.023 

Hornblende. 

713 

Greenish  blue. 

0.024 

Diallage. 

747 

Green. 

0.025 

Actinolite.    Augite. 

810 

Light  green. 

0.027 

Tremolite,  arfvedsonite. 

855 

Yellowish  green. 

0.029 

Diopside,  cancrinite. 

46 


OPTICAL  MINERALOGY  AND  PETROGRAPHY 


Millionths 

of  a  Interference  of  Colors 

mm   Re- 
tardation 

1079 

1228 


1140 
1260 
1300 
1425 
1495 
1652 

1845 
2170 
2900 
3600 
4600 
5200 
5400 
6100 
7200 
8400 
8600 


Between   X   Nicols. 

g    p 

Dark  orange  violet. 

0.036 

Violet  (Sensitive  tint 

No.  2). 

0.037 

Indigo. 

0.038 

Greenish  blue. 

0.042 

Sea  green. 

0.044 

Greenish  yellow. 

0.048 

Rose  red. 

0.050 

Violet  gray   (Sensi- 

tive tint  No.  3). 

0.056 

Greenish  gray. 

0.062 

Colors  very  faint. 

0.072 

0.097, 

0.121 

• 

0.155 

Not  distinguishable. 

0.172 

0.179 

0.202 

0.239 

0.280 

With  shades  of  red 

and  green. 

0.287 

Rock-forming 
Minerals. 


Olivine,  lazurite. 


Epidote. 

Muscovite. 

Phlogopite,  anhydrite. 

Limonite. 

Talc. 


Zircon. 

Hornblende. 

Cassiterite. 

Titanite. 

Aragonite. 

Oalcite. 

Dolomite. 

Magnesite. 

Siderite. 

Hematite. 

Rutile. 


The  lowest  colors  of  the  above  scale  are  the  colors 
of  the  first  order,  which  includes  all  of  the  colors  up  to 
the  first  violet,  which  marks  the  limit  of  the  order.  The 
colors  of  the  second  and  third  orders  are  successively 
higher.  In  the  fourth  order,  the  colors  begin  to  ap- 
proach white  light,  due  to  an  overlapping  of  the  inter- 
ference. The  highest  color  which  the  mineral  is  capable 
of  producing  is  usually  taken  for  comparison  with  the 
colors  of  the  table.  In  uniaxial  minerals,  such  colors  are 
given  by  sections  cut  parallel  to  the  optic  axis. 

Determination  of  the  Order  of  Color  Produced  by  In- 
terference.— The  rank  of  an  interference  color  may  be 


METHODS  OF    MINERAL  DETERMINATION  47 

determined  by  means  of  a  "quartz  wedge."  This  is  a 
quartz  plate  of  varying  thickness,  which  gives  the  colors 
of  the  Newton  scale  from  the  grayish  blue  of  the  first 
order  up.  The  quartz  wedge  is  mounted  on  a  plate 
of  gypsum  in  such  a  position  that  the  faster  ray  in  the 
gypsum  is  the  slower  ray  in  the  quartz.  The  gypsum 
plate  is  made  of  such  thickness  that  its  effect  is  com- 
pletely compensated  by  that  of  the  wedge  at  the  middle 
of  the  latter.  Between  crossed  nicols,  darkness  will  re- 
sult at  this  point.  Upon  moving  the  wedge  in  either 
direction,  the  colors  rise  successively  from  this  zero  or 
compensating  point  to  colors  of  the  third  order.  When 
the  wedge  is  superimposed  over  the  thin  section  of  a 
mineral,  the  colors  rise  in  the  scale  if  moved  in  one  di- 
rection and  fall  if  moved  in  the  other  direction. 

Assume  that  a  mineral  is  placed  upon  the  stage  of 
a  microscope  between  crossed  nicols  and  in  its  position 
of  maximum  illumination.  Insert  a  quartz  wedge  in 
the  proper  slit  in  the  microscope  tube  above  the  thin  sec- 
tion, and  note  the  change  of  colors  as  the  wedge  is  moved 
over  the  field,  the  thin  edge  being  inserted  first.  If  the 
colors  rise  in  the  scale  from  yellow  to  red  to  violet  to 
blue  to  green  and  again  to  yellow,  it  is  an  indication  that 
the  greater  axis  of  ether  vibration  of  the  thin  section 
and  that  of  the  quartz  wedge  are  parallel.  Therefore, 
turn  the  stage  90  degrees  to  its  former  position  and  in- 
sert the  wedge  again.  The  order  of  the  change  of  colors 
will  now  be  reversed.  The  colors  will  fall,  indicating 
that  the  lesser  axis  of  ether  vibration  of  the  thin  section 
is  parallel  to  the  greater  axis  of  the  wedge.  Move  the 
wedge  over  the  mineral  until  the  plate  becomes  dark  or 
gray.  This  is  the  "compensation  point,"  where  the  accel- 
eration of  one  of  the  rays  of  the  plate  corresponds  ex- 
actly to  the  retardation  of  the  same  in  the  wedge.  Re- 
move the  mineral  from  the  stage.  The  interference  color 


48  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

that  the  wedge  displays  is  now  the  same  as  that  orig- 
inally shown  by  the  mineral.  Slowly  remove  the  wedge, 
observing  carefully  the  sequence  of  colors.  The  number 
of  times  that  any  color  recurs  until  the  wedge  is  re- 
moved gives  the  order  of  the  original  interference  color 
of  the  mineral. 

From  a  birefringence  chart  it  is  possible  to  deter- 
mine not  only  the  order  of  birefringence  of  a  mineral 
but  the  thickness  of  the  section,  provided  some  mineral 
contained  in  the  slide  is  known.  Let  us  take,  as  an  ex- 
ample, granite  in  which  quartz  is  easily  recognized.  It 
is  fairly  safe  to  assume  that,  if  there  are  many  frag- 
ments of  quartz  in  the  field  of  view,  the  fragment  with 
the  highest  interference  color  is  cut  parallel  to  the  optic 
axis,  and  its  birefringence  has  a  maximum  value,  0.009. 
This  value  on  the  color  chart  is  marked  by  a  diagonal 
line,  which  should  be  followed  toward  the  lower  left- 
hand  corner  to  the  intersection  with  the  vertical,  giv- 
ing the  interference  color  shown  in  the  slide.  The 
ordinate  at  the  point  of  intersection  represents  the  thick- 
ness of  the  section.  Its  value  is  determined  by  follow- 
ing the  horizontal  line  through  the  intersection  to  the 
scale  on  the  left.  This  reading  gives  the  thickness  of 
the  section  in  millimeters. 

Having  thus  determined  the  thickness  of  the  sec- 
tion, find  again  the  highest  interference  color  in  a  frag- 
ment of  the  mineral  which  is  to  be  determined.  Take 
the  intersection  of  the  horizontal  line  of  thickness  in 
the  chart  with  this  color.  The  diagonal  line  passing 
through  this  point  of  intersection  indicates  the  birefrin- 
gence of  the  mineral  in  question. 

Double  Refraction  of  Biaxial  Minerals.— Minerals  of 
the  orthorhombic,  monoclinic  and  triclinic  systems. 

Minerals  of  these  three  systems  have  two  optic  axes, 


METHODS  OF    MINERAL  DETERMINATION  49 

or  two  directions  in  which  light  is  singly  refracting; 
hence  the  name  biaxial.  Sections  cut  perpendicular  to 
these  directions  remain  dark  between  crossed  nicols  dur- 
ing a  complete  revolution  of  the  stage.  The  optic  axes 
of  biaxial  minerals  never  coincide  in  position  with  any 
of  the  crystallographic  axes  as  is  true  in  the  case  of  uni- 
axial  minerals.  In  the  orthorhombic  system  they  lie  in 
the  same  plane  with  two  of  these  axes. 

Biaxial  minerals  contain  greatest  and  least  axes  of 
ether  elasticity,  which,  as  in  uniaxial  minerals,  are 
denoted  by  X  and  Z.  In  addition,  there  is  a  mean  axis 
of  ether  elasticity,  denoted  by  Y. 

OPTIC  AXIAL  PLANE.  The  plane  containing  X  and  Z 
also  contains  the  two  optic  axes,  and  is  called  the  optic 
axial  plane,  or  the  optic  plane.  The  mean  axis  of  ether 
elasticity,  Y,  is  normal  to  this  plane,  and  is  called  the 
optic  normal. 

BISECTRICES.  The  optic  axes  intersect  each  other  at 
the  point  of  intersection  of  the  optic  plane  with  the  other 
planes  of  symmetry,  if  any  exist,  making  equal  angles 
on  opposite  sides  of  the  axes  X  and  Z.  Therefore,  X 
and  Z  are  known  as  bisectrices.  When  X  bisects  the 
acute  angle  of  the  optic  axes,  it  is  called  the  acute  bi- 
sectrix. The  same  is  true  for  Z.  When  they  bisect  the 
obtuse  angle,  they  are  called  obtuse  bisectrices. 

Orthorhombic  minerals  have  three  axes  of  ether  vi- 
bration parallel  to  the  crystallographic  axes.  The  direc- 
tion of  X  may  be  the  same  as  a,  b  or  c,  the  directions 
of  Y  and  Z  varying  accordingly,  but  always  at  right 
angles  to  each  other. 

In  monoclinic  minerals,  one  of  the  axes  of  ether  vi- 
bration, frequently  Y,  coincides  with  the  crystallographic 
axis  b  (the  axis  of  symmetry),  and  the  other  two  are 
in  the  plane  of  symmetry,  parallel  to  the  clinopinacoid. 

In  the  triclinic  system,  the  axes  of  ether  vibration 


50 


OPTICAL  MINERALOGY  AND  PETROGRAPHY 


have  no  fixed  relation  to  the  crystallographic  axes. 

The  discussion  of  the  determination  of  birefringence 
of  uniaxial  minerals  is  applicable  to  biaxial  minerals. 
The  interference  colors  in  sections  of  biaxial  minerals 
normal  to  the  optic  elements  grade  downward  in  the 
following  order  from  highest  to  lowest: 

1.  Optic  normal. 

2.  Obtuse  bisectrix. 

3.  Acute  bisectrix. 

4.  Optic  axis. 

The  following  scale  of  birefringence  (after  Win- 
chell)  is  useful  for  comparison  in  the  estimation  of  the 
birefringence  of  an  unknown  mineral. 


SCALE  OF  BIREFRINGENCE. 


1.  Very  weak  birefringence 

2.  Weak  birefringence 

3.  Moderate  birefringence 

4.  Rather  strong  birefrin- 

gence 

5.  Strong  birefringence 

6.  Strong  birefringence 

7.  Very  strong  birefringence 

8.  Extreme  birefringence 


0.0035  or  less.  Example,  leucite. 

0.0035-0.0095.  Example,  orthoclase. 

0.0095-0.0185.  Example,  hypersthene. 

0.0185-0.0275.  Example,  augite. 

0.0275-0.0355.  Example,  diopside. 

0.0355-0.0445.  Example,  muscovite. 

0.0445-0.0565.  Example,  aegirite. 

0.0565.  Example,  titanite. 


METHODS   OF   MINERAL  DETERMINATION     .  51 


CHAPTER  4. 
General  Methods  of  Mineral  Determination  (Continued). 

General  Operation  No.  4:  Determination  of  the  Axial 
Interference  Figures. 

Interference  figures  are  obtained  by  the  use  of  crossed 
nicols  in  convergent  light.  A  high-power  objective  must 
be  used.  When  the  eyepiece  is  removed,  a  small  image 
of  the  interference  figure  can  be  seen.  By  sliding  the 
Bertrand  lens  into  the  tube  of  the  microscope,  a  mag- 
nified image  of  the  figure  is  obtained,  in  which  case  the 
ocular  is  retained.  Strong  illumination  is  necessary, 
with  the  condensing  lens  close  under  the  thin  section. 
Results  are  best  with  monochromatic  light,  but  the  ef- 
fects are  the  same  with  white  light  except  that  the  rings 
will  be  variously  colored  instead  of  light  and  dark. 

This  operation  aids  the  observer  in  distinguishing 
between  isotropic,  uniaxial  and  biaxial  substances,  and 
aids  in  the  determination  of  the  relative  double  refrac- 
tion of  minerals. 

Isotropic  minerals  show  no  interference  figures. 

UNIAXIAL  INTERFERENCE  FIGURES: 

a.  Sections  cut  perpendicular  to  the  optic  axis  or 
vertical  crystallographic  axis  show  a  dark  cross  with  or 
without  colored  rings.    The  arms  of  the  cross  are  parallel 
to  the  vibration  planes  of  the  nicols,  and  the  figure  does 
not  move  with  the  rotation  of  the  section. 

b.  Sections  cut  oblique  to  this  position  show  figures 
which  move  about  the  center  of  the  field.     The  center 
of  the  figure  may  even  be  outside  of  the  field,  but  upon 


52  •    OPTICAL  MINERALOGY  AND  PETROGRAPHY 

rotation  its  dark  bars  may  be  seen  to  move  across  the 
field.  These  dark  bars  remain  straight  and  parallel  to 
themselves. 


Fi*.  8.     Uniaxial  figure. 

If  the  obliquity  of  the  section  is  too  great,  the  bars 
will  show  a  curvature  upon  entering  the  field  and  upon 
leaving,  but  they  are  straight  upon  crossing  the  center 
of  the  field.  The  curvature  shifts  upon  crossing  the  cen- 
ter from  one  side  to  the  other,  thereby  differing  from 
the  biaxial  figures,  in  which  the  bars  remain  curved  in 
the  same  direction. 

c.  If  the  section  becomes  so  oblique  to  the  optic  axis 
as  to  approach  parallelism  to  it,  the  black  cross  appears 
to  break  up  into  hyperbolas  which  are  symmetrically 
placed  with  respect  to  the  optic  axes,  and  then  unite  to 
form  a  dark  cross  again  upon  completing  the  rotation  of 
the  section. 

Sections  which  are  thick  and  have  a  strong  double  re- 
fraction show  the  cross  and  rings  clearly  and  sharply 
outlined,  many  rings  being  crowded  closely  together. 
Thin  sections  with  weak  double  refraction  show  broad 
crosses  and  no  rings.  The  observer  may  thus  deduce 
inferences  both  in  regard  to  the  thickness  of  the  section 
and  the  strength  of  the  double  refraction. 

BIAXIAL  INTERFERENCE  FIGURES: 

a.  Sections  cut  normal  to  an  optic  axis  show  a  series 
of  concentric  colored  curves  crossed  by  a  single  dark 
bar.  The  bar  changes  into  a  hyperbola  and  back  into  a 


METHODS  OF   MINERAL  DETERMINATION 


53 


bar.  Sometimes  the  curves  are  not  observable.  The  bar 
when  straight  shows  the  direction  of  the  optic  plane  with 
which  it  is  parallel. 


m 


b.  Sections  cut  normal  to  the  acute  bisectrix  in  which 
the  angle  between  the  optic  axes  is  not  too  great  will 
show  both  optic  axes  in  the  interference  figure,  the  bisec- 
trix being  in  the  center  of  the  field  between  them.  In 


54 


OPTICAL  MINERALOGY  AND  PETROGRAPHY 


one  case  a  dark  bar  appears  in  the  center  of  the  field,  its 
arms  varying  in  size.  That  line  which  passes  through  the 
optic  axes  is  narrower  than  the  one  passing  between 


Fig.  10.     Optic  axis 
interference  figure. 

them.  Its  extremities  widen  out  on  the  edge  of  the  field. 
The  intersection  of  the  two  bars  marks  the  bisectrix. 
The  trace  of  the  optic  plane  is  the  line  passing  through 
the  loci  of  the  optic  axes  and  the  bisectrix.  On  rotating 
the  section,  the  dark  bars  separate  into  two  hyperbolas, 
the  summits  receding  from  each  other  toward  the  edge 
of  the  field,  and  beyond  it  if  the  optic  axes  are  not  in 
view.  They  bend  through  the  colored  curves  surround- 
ing the  optic  axes,  and  unite  again  as  a  straight  bar  when 


Fig.    11.      Bisectrix   interference 
figure. 


Fig.   12.     Bisectrix  interference 
figure  at  45°. 


the  plane  of  the  optic  axes  coincides  with  the  vibration 
plane  of  the  nicol.  The  most  distant  positions  of  the 
hyperbolic  summits  are,  therefore,  after  a  revolution  of 
45  degrees. 


METHODS   OF    MINERAL  DETERMINATION 


55 


An  excellent  illustration  of  the  biaxial  interference 
figure  may  be  obtained  very  simply  by  placing  between 
crossed  nicols  in  convergent  light  a  thin  sheet  of  musco- 


€) 


CD 


•- 


vite  mica.  Since  the  optic  angle  is  small,  the  loci  of  both 
optic  axes  will  be  seen  in  the  field.  Since  the  center  of 
the  small  ellipses  and  the  black  hyperbolas  mark  the  loci 


56  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

of  the  optic  axes,  they  indicate  approximately  the  optic 
angle. 

The  uniaxial  or  biaxial  character  of  a  mineral  sec- 
tion which  shows  only  an  indistinct  bar  may  be  deter- 
mined as  follows  (La  Croix)  : 

A  bar  of  a  uniaxial  interference  figure  moves  in  the 
same  direction  as  the  rotating  stage,  and  always  remains 
straight,  while  the  biaxial  bar  moves  in  the  opposite 
direction  to  that  of  the  stage,  and  becomes  curved. 

General  Operation  No.  5:  Determination  of  the  Dis- 
persion of  the  Optic  Axes. 

The  colors  of  the  interference  figures  in  convergent 
light  are  caused  by  the  difference  of  phase  of  different 
rays  brought  together  by  the  analyzer  so  as  to  inter- 
fere. The  phenomenon  of  relative  position  of  the 
red  and  violet  rays  is  caused,  by  the  dispersion  of  the  optic 
axes.  When  the  red  ray  has  the  greater  optic  angle  it  is 
expressed  by  R  >  V ;  when  the  violet  ray  has  the  greater 
optic  angle,  it  is  expressed  by  R  <  V. 

When  white  light  is  used,  the  colors  on  the  convex 
side  of  the  hyperbola  (which  is  the  side  toward  the  acute 
bisectrix)  are  edged  with  red  if  the  dispersion  of  red  is 
greater  than  that  of  violet,  and  edged  with  violet  if  the 
reverse  is  true. 

Labradorite,  muscovite,  orthoclase,  and  anorthite 
have  a  dispersion  formula  R  >  V.  Albite  and  oligoclase 
have  a  dispersion  formula  V  >  R. 

General  Operation  No.  6:  Determination  of  the  Op- 
tical Character  or  the  Character  of  the  Double  Refraction 

(after  Winchell)  : 

OPTICAL  CHARACTER  OF  UNIAXIAL  MINERALS.  For 
light  traveling  perpendicular  to  the  optic  or  vertical  crys- 
tallographic  axis,  the  vibrations  of  the  ordinary  ray  are 
transverse  to  that  axis  and  those  of  the  extraordinary 


METHODS   OF   MINERAL  DETERMINATION 


57 


ray  are  parallel  to  it;  o  may  be  greater  or  less  than  e, 
as  the  vertical  axis  may  be  greater  or  less  than  the  hori- 
zontal crystallographic  axes. 

If  the  vertical  axis  is  the  direction  of  the  greater 
axis  of  ether  vibration  (X) ,  the  mineral  is  optically  nega- 
tive. The  extraordinary  ray  is  less  refracted  than  the 
ordinary  ray,  and  advances  with  greater  velocity.  This 
is  expressed  as  o  >  e.  Optically  positive  minerals  are 
those  in  which  o  <  e.  The  greater  the  velocity,  the  less 
the  refraction,  and  the  smaller  the  index  of  refraction. 

To  determine  the  sign  of  an  unknown  mineral,  one 
must  be  able  to  compare  the  relative  velocities  of  the 
ordinary  ray  (vibrating  perpendicular  to  the  primary 


°l 

•f 

•    ° 

Fig.    16.     Quarter-undulation   mica   plate. 

axis)  and  the  extraordinary  ray  (vibrating  parallel  to 
the  primary  axis)  within  the  thin  section,  with  the 
known  velocities  of  another  mineral.  This  may  be  ac- 
complished by  the  following  methods : 

A.  With  the  quarter-undulation  mica  plate  in  parallel 
polarized  light. 

The  quarter-undulation  mica  plate  consists  of  a  cleav- 
age leaf  of  muscovite,  the  thickness  of  which  is  just 
sufficient  to  produce  a  retardation  of  a  quarter  of  a  wave- 
length of  light,  or  about  two  of  the  larger  vertical  divi- 
sions of  the  color  chart.  It  is  mounted  between  .two  glass 
plates.  An  arrow  on  the  glass  usually  indicates  the  direc- 
tion of  the  lesser  axis  of  ether  vibration  (Z). 

When  the  coinciding  axes  of  the  mica  plate  and  the 


58  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

mineral  are  the  same  (Z),  the  double  refraction  is  in- 
creased in  proportion  to  the  resultant  thickness  of  the  two 
plates.  The  color  of  the  section  rises  through  two  of  the 
vertical  divisions  of  the  color  chart.  The  color  falls  cor- 
respondingly if  the  axes  are  not  the  same. 

B.  With  the  quartz-sensitive  tint  in  polarized  light. 

The  quartz-sensitive  tint  is  a  plate  of  quartz  cut 
parallel  to  its  vertical  or  lesser  axis  of  ether  vibration, 
and  is  of  such  a  thickness  as  to  give  the  first  violet  color 
of  Newton's  scale.  It  is  mounted  between  glass  plates. 
The  direction  of  the  vertical  crystallographic  axis,  as  well 

In   parallel   polarized   light. 


r~o\ 

> 

QUARTZ  PLATE            \^.2£\ 
Sensitive  tint                           %    ^"-N 

€>W                                                         \lL     \ 

vjE  <  CO       / 

Color  falls :     Negative. 
Fig:.   17.     Use  of  the  quartz-sensitive  tint. 

as  the  optic  axis,  corresponds  with  the  direction  of  Z, 
and  is  usually  indicated  by  an  arrow  on  the  glass. 

The  position  of  the  axes  of  ether  vibration  in  the  un- 
known mineral  is  first  determined.  This  is  done  by  deter- 
mining on  rotation  between  crossed  nicols,  the  positions 
at  which  extinction  takes  place.  The  section  is  placed 
45  degrees  to  this  position.  The  brightest  interference 
color  is  thus  produced.  Place  the  quartz-sensitive  tint 
over  the  section  in  such  a  position  that  the  direction  of  Z 
is  45  degrees  with  the  principal  sections  of  the  nicols. 
If  by  this  superposition  a  color  is  produced  which  is 
higher  in  the  scale  than  the  sensitive  tint  of  the  quartz 


METHODS   OF   MINERAL  DETERMINATION  59 

plate,  the  axis  Z  of  the  quartz  plate  is  parallel  to  the  axis 
Z  of  the  thin  section.  If  the  resultant  color  is  lower, 
the  axis  of  ether  vibration  of  the  mineral  is  X,  in  conse- 
quence of  a  lessening  of  the  retardation. 

Uniaxial  minerals  are  positive  when  Z  coincides  with 
the  optic  axis,  and  negative  when  X  does.  Since  the 
optic  axis  coincides  with  the  vertical  crystallographic 
axis,  it  is  necessary  when  using  this  method  to  be  able  by 
the  crystal  outline  to  determine  the  direction  of  the 
vertical  axis.  This  method  is,  therefore,  practicable  only 
when  the  section  is  approximately  parallel  with  the  ver- 
tical axis  and  the  crystal  outline  is  distinct. 

C.  With  the  quartz  wedge  in  parallel  polarized  light. 
This  method  is  the  same  as  method  B. 


Fie.  I*.     Disturbance  of  the  interference  figure  of  a  uniaxial 
crystal  by  the  quarter-undulation  mica  plate. 

D.  With  the  quarter-undulation  mica  plate  in  conver- 
gent light. 

By  inserting  the  plate  with  its  Z  axis  45  degrees  with 
the  cross  hairs,  the  dark  cross  of  the  interference  figure 
is  destroyed  and  two  dark  spots  are  brought  prominently 
into  view.  If  rings  are  seen,  they  will  appear  disjointed 
at  the  lines  dividing  the  quadrants,  and  they  will  appear 
expanded  in  those  quadrants  occupied  by  the  dark  spots. 

The  mineral  is  optically  positive  if  a  line  joining  the 
two  dark  spots  is  perpendicular  to  the  axis  of  the  mica 
plate.  The  mineral  is  negative  if  the  line  uniting  the 


60  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

dark  spots  is  parallel  with  the  direction  of  the  arrow  on 
the  mica  plate. 

The  positive  and  negative  character  of  the  mineral 
becomes  a  simple  operation  if  it  is  borne  in  mind  that 
the  line  joining  the  dark  spots  makes  a  positive  sign  and 
a  negative  sign  respectively  with  the  axis  of  the  mica 
plate,  thereby  indicating  directly  the  sign  of  the  mineral. 

E.  With  the  quartz-sensitive  tint  in  convergent  light. 

Upon  inserting  the  sensitive  tint  plate,  two  opposite 
quadrants  will  appear  yellow  and  the  other  set  will  ap- 
pear blue.  In  determining  the  sign  of  the  mineral,  the 
yellow  quadrants  may  be  considered  equivalent  to  the 
dark  spots. 

When  a  section  is  cut  parallel  with  the  optic  axis, 
the  interference  figure  is  not  a  black  cross  but  may  re- 
semble a  biaxial  interference  figure.  The  observer  wishes 
to  determine  the  direction  of  the  optic  axis.  He  may 
determine  this  by  observing  in  which  quadrants  the 
hyperbolas  always  leave  the  field.  These  will  be  the  quad- 
rants containing  the  optic  axis.  Moreover,  the  inter- 
ference colors  in  these  quadrants  are  lower  than  for 
corresponding  points  in  the  other  quadrants.  After 
once  determining  the  direction  of  the  optic  axis,  Z  can  be 
determined  by  any  one  of  the  first  three  methods  for 
parallel  polarized  light. 

OPTICAL  CHARACTER  OF  BIAXIAL  MINERALS.  If  the 
greatest  axis  of  ether  vibration  bisects  the  acute  bisec- 
trix, the  mineral  is  negative.  If  Z  bisects  the  acute  bisec- 
trix, the  mineral  is  positive.  Therefore,  a  determination 
of  the  optic  sign  of  a  biaxial  mineral  demands  a  distinc- 
tion of  the  acute  from  the  obtuse  bisectrix  and  a  distinc- 
tion of  X  and  Z. 

Distinction  between  the  Acute  and  Obtuse  Bisectrices. 
The  thin  section  is  cut  perpendicular  to  the  acute  bisec- 


METHODS   OF    MINERAL  DETERMINATION  61 

trix  if  the  optic  angle  is  so  small  that  the  loci  of  the 
optic  axes  or  of  one  optic  axis  and  the  bisectrix  remain 
in  the  field  during  a  rotation  of  the  stage.  Otherwise 
it  is  necessary  to  find  sections  cut  perpendicular  to  both 
X  and  Z  and  compare  them. 

1.  The  section  perpendicular  to  the  acute  bisectrix 
shows  a  lower  interference  color  than  the  section  cut 
perpendicular  to  the  obtuse  bisectrix. 

2.  The  angle  of  rotation  between  the  position  of  the 
black  cross  and  the  position  when  the  summits  of  the 
hyperbolas  are  tangent  to  the  edge  of  the  field  can  be 
measured.    This  angle  is  larger  in  a  section  perpendicu- 
lar to  an  acute  bisectrix  than  in  one  perpendicular  to 
the  obtuse  bisectrix.    If  the  angle  is  more  than  30  or  35 
degrees,  it  is  safe  to  assume  that  the  section  is  perpen- 
dicular to  an  acute  bisectrix.    If  the  angle  is  less  than  15 
or  20  degrees,  it  is  perpendicular  to  an  obtuse  bisectrix. 

Distinction  between  X  and  Z.  This  involves  a  com- 
parison of  the  velocities  of  the  light  ray  in  the  direction 
of  the  axis  to  be  determined  with  that  in  the  direction  of 
a  known  velocity  in  another  mineral.  Relative  retarda- 
tion is  indicated  by  the  relative  positions  of  the  colors  on 
Newton's  scale.  The  following  methods  are  available: 

A.  With  the  quartz-sensitive  tint  in  parallel  polar- 
ized light. 

The  section  examined  must  be  parallel  to  the  optic 
plane,  that  is,  it  must  contain  the  axes  Z  and  X.  Z  of  the 
quartz  plate  lies  in  the  direction  of  the  arrow,  and  X  at 
right  angles  to  this  direction.  Superpose  the  quartz  plate 
over  the  mineral.  If  the  resultant  color  is  higher  than 
the  sensitive  tint  of  the  quartz  plate,  the  Z  axes  of  the 
quartz  plate  and  of  the  mineral  are  coincident.  If  the 
resultant  color  is  lower,  the  Z  axis  of  the  quartz  plate  is 
coincident  with  the  X  axis  of  the  mineral.  This  enables 


62  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

the  observer  to  determine  the  position  on  the  two  axes. 

It  is  now  necessary  to  view  the  interference  figure 
in  convergent  light  in  order  to  determine  in  which  quad- 
rants the  optic  plane  lies.  If  Z  is  found  to  lie  in  the  acute 
optic  angle,  it  bisects  the  acute  bisectrix,  and  the  mineral 
is  positive. 

If  only  two  hyperbolas  are  observed,  they  are  in  the 
quadrants  containing  the  acute  bisectrix.  If  the  optic 
angle  is  large,  hyperbolas  may  be  visible  in  all  four  quad- 
rants, but  the  hyperbolas  leave  the  field  more  slowly  in 
the  quadrants  containing  the  acute  bisectrix. 

The  quarter-undulation  mica  plate  may  be  used  in 
the  same  manner  as  with  uniaxial  minerals  to  determine 
the  directions  of  X  and  Z  since  the  section  is  cut  parallel 
to  the  plane  containing  X  and  Z. 

B.  With  the  quartz  wedge  in  convergent  light. 

Obtain  an  interference  figure  from  a  section  as  nearly 
normal  to  the  acute  bisectrix  as  possible,  and  rotate  the 
stage  until  the  optic  plane  makes  a  45-degree  angle  with 
the  vibration  planes  of  the  nicols. 

Insert  the  quartz  wedge,  with  the  thin  edge  advanced, 
in  such  a  position  that  the  Z  axis  coincides  in  direction 
with  a  line  passing  through  the  optic  axes  of  the  figure. 

The  optical  character  of  the  mineral  is  positive  when 
the  ellipses  surrounding  the  loci  of  the  optic  axes  appear 
to  widen  out,  and  move  from  the  loci  of  the  optic  axes 
toward  the  center  of  the  interference  figure  and  finally 
open  into  the  outer  colored  margins  surrounding  the 
whole  figure.  The  optical  character  of  the  mineral  is 
negative  if  the  movement  of  the  colors  is  reversed  from 
the  center  of  the  figure  toward  the  axial  spots. 

With  certain  interference  figures  the  following  simple 
rule  will  apply:  If  the  dark  spots  approach  each  other, 
the  mineral  is  negative.  If  they  appear  to  retreat  from 
each  other,  the  mineral  is  positive. 


METHODS  OF   MINERAL  DETERMINATION  63 

C.  With  the  quarter-undulation  mica  plate  in  con- 
vergent light. 

This  plate  is  perpendicular  to  the  negative  bisectrix 
X,  and  contains  Z  and  Y.  The  direction  of  Z  coincides 
with  the  trace  of  the  plane  of  the  optic  axes,  since  the 
axial  plane  always  contains  X  and  Z. 

For  sections  perpendicular  to  an  acute  bisectrix. 
When  the  mica  plate  is  superposed  in  the  usual  way,  there 
is  an  apparent  lengthening  of  the  figure  in  the  direction 
of  the  Z  axis  of  the  mica  plate  and  an  apparent  short- 
ening in  this  direction  for  positive  minerals.  Winchell 
suggests  that  this  observation  be  made  with  the  optic 
plane  parallel  with  one  nicol.  The  dark  spots  will  now 
appear  in  the  quadrants  through  which  the  arrow  passes, 
the  line  connecting  them  forming  an  angle  less  than  45 
degrees  with  the  arrow,  or  an  approximate  minus  sign 
indicating  a  negative  mineral.  The  reverse  takes  place 
with  a  positive  mineral.  There  is  a  shortening  in  the 
direction  of  the  arrow,  and  the  line  connecting  the  dark 
spots  forms  an  angle  greater  than  45  degrees  with  the 
arrow,  making  an  approximate  plus  sign. 

Iddings  suggests  the  following  method  for  sections 
perpendicular  to  an  optic  axis :  Place  the  section  with  its 
optic  plane  45  degrees  with  the  nicols.  The  hyperbola  is 
convex  toward  the  acute  bisectrix.  Insert  the  mica  plate 
with  the  Z  axis  parallel  with  the  optic  plane  of  the  min- 
eral. The  hyperbola  moves  toward  the  obtuse  bisectrix 
when  the  mineral  is  negative,  and  toward  the  acute  bisec- 
trix when  the  mineral  is  positive.  For  minerals  of  weak 
birefringence,  as  the  feldspars,  this  method  is  excellent. 

For  minerals  of  strong  birefringence  the  following 
rule  may  be  applied:  The  mineral  is  positive  when  the 
black  dot  appears  on  the  convex  side  of  the  hyperbola 
upon  insertion  of  the  mica  plate  with  its  Z  axis  parallel 
with  the  optic  plane  of  the  mineral. 


64          OPTICAL  MINERALOGY  AND  PETROGRAPHY 

SUMMARY  OF  THE  OPTICAL  SIGN  —  FOR  UNIAXIAL 
MINERALS.  When  the  E  ray  is  less  refracted  than  the  O 
ray  and  advances  with  greater  velocity,  the  mineral  is 
negative,  as  in  calcite.  In  this  case,  X  coincides  with  the 
optic  or  vertical  axis.  The  index  of  refraction  for  the  E 
ray  vibrating  in  this  direction  is  the  lesser  one,  rip. 

When  the  0  ray  is  less  refracted  than  the  E  ray  and 
advances  with  the  greater  velocity,  the  mineral  is  posi- 
tive, as  in  quartz.  In  this  case,  Z  coincides  with  the  verti- 
cal or  optic  axis.  The  index  of  refraction  of  the  E  ray 
which  is  vibrating  in  this  direction  is  the  greater  one,  ng. 

FOR  BIAXIAL  MINERALS.  When  X  is  the  acute  bisec- 
trix, the  mineral  is  negative,  as  in  muscovite. 

When  Z  is  the  acute  bisectrix,  the  mineral  is  posi- 
tive, as  in  augite. 

General  Operation  No.  7:  Determination  of  the  Ex- 
tinction Angle,  or  the  Relation  of  the  Grystallographic 
Axes  to  the  Axes  of  Ether  Vibration. 

This  operation  is  performed  between  crossed  nicols 
with  parallel  polarized  light. 

It  will  be  remembered  that  the  intensity  of  color  de- 
pends upon  the  angle  between  the  principal  optic  sec- 
tion of  the  mineral  and  the  principal  section  of  either 
nicol,  the  color  being  greatest  at  45  degrees  and  least 
at  0  degrees.  Thus  when  the  section  is  in  such  a  posi- 
tion that  its  directions  of  elasticity  are  parallel  to  the 
vibration  planes  of  the  nicols,  no  light  can  pass  through 
the  analyzer,  and  the  section  is  dark.  This  phenomenon 
is  called  extinction. 

Extinction  is  the  most  common  phenomenon  for  dis- 
tinguishing isotropic  minerals  from  anisotropic.  In 
the  isometric  system  all  minerals  are  completely  dark 
during  a  rotation  of  the  stage.  It  is  likewise  of  great 


METHODS  OF   MINERAL  DETERMINATION  65 

importance  in  distinguishing  between  minerals  of  the 
three  biaxial  systems. 

Extinction  is  said  to  be  parallel  when  the  directions 
of  the  axes  of  ether  elasticity  are  parallel  to  any  crys- 
tallographic  directions,  which  may  be  determined  by 
cleavage,  crystal  boundaries,  or  twinning  lines. 

Parallel  extinction  is  shown  by  all  sections  of  tet- 
ragonal, hexagonal,  orthorhombic  minerals,  and  in  the 
monoclinic  minerals  in  sections  parallel  to  the  b  axis 
or  orthozone.  Oblique  extinction  is  shown  in  all  other 
sections  of  the  monoclinic  minerals  and  all  sections  of 
triclinic  minerals.  In  the  triclinic  system  there  is  no 
coincidence  between  the  axes  of  elasticity  and  the  crys- 
tallographic  axes. 

The  angle  between  an  axis  of  elasticity  in  the  sec- 
tion and  some  known  crystallographic  direction  is  called 
the  extinction  angle.  It  is  measured  as  follows :  Find 
the  positions  of  the  axes  of  elasticity  in  the  section  when 
extinction  takes  place.  Note  the  reading  on  the  vernier 
of  the  stage.  Rotate  the  vernier  until  the  known  crys- 
tallographic direction  is  brought  into  a  parallel  posi- 
tion with  the  same  cross  hair  which  was  previously  used. 
A  more  distinct  view  of  the  field  may  be  obtained  by 
removing  the  upper  nicol.  The  difference  between  these 
two  readings  is  the  extinction  angle. 

In  monoclinic  minerals  the  maximum  value  of  the 
extinction  angle,  which  is  the  only  angle  of  real  value 
in  differentiating  the  mineral,  is  obtained  from  a  section 
parallel  to  the  clinopinacoid.  Results  accurate  enough 
for  all  practical  purposes  may  be  obtained  by  measuring 
the  angle  of  all  sections  of  the  mineral  and  taking  the 
maximum  value  obtained. 

Amphiboles  and  pyroxenes  are  easily  distinguished  in 
this  manner. 

Extinction  which  passes  over  the  section  like  a  dark 


66  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

wave  or  shadow  is  called  undulatory  extinction.  It  in- 
dicates that  the  mineral  has  been  subjected  to  mechanical 
forces,  producing  a  change  in  the  position  of  the  axes 
of  elasticity  in  different  parts  of  the  mineral. 

It  is  difficult  for  the  eye  to  distinguish  small  varia- 
tions in  the  intensity  of  light.  By  the  use  of  the  quartz- 
sensitive  tint,  extinction  is  determined  quite  accurately 
by  a  distinction  of  difference  of  color  to  which  the  eye 
is  more  susceptible.  A  thin  quartz  plate  is  cut  parallel 
to  the  axis  of  elasticity,  having  such  a  thickness  that 
it  shows  the  violet  color  of  Newton's  scale.  Insert  this 
plate  in  such  a  position  that  its  axis  is  45  degrees  to 
the  cross  hairs.  The  field  of  the  microscope  is  violet. 
By  placing  the  unknown  mineral  on  the  stage  so  as  not 
to  occupy  the  entire  field,  it  will  be  seen  that  the  color 
of  the  mineral  is  not  the  same  as  the  violet  color  of 
the  unoccupied  field.  Rotate  the  stage  until  the  color 
of  the  mineral  is  the  violet  color  of  the  quartz  plate. 
The  mineral  is  now  at  extinction.  This  phenomenon  is 
due  to  the  fact  that  the  axes  of  elasticity  of  the  nicols 
and  of  the  mineral  are  in  the  same  position  and  producing 
no  interference. 

General  Operation  No.  8:  Determination  of  the  Pres- 
ence or  Absence  of  Pleochroism. 

Pleochroism  is  a  property  possessed  by  all  aniso- 
tropic  minerals  of  absorbing  certain  colored  rays  in  cer- 
tain crystallographic  directions,  thereby  showing  differ- 
ent colors  in  different  directions  by  transmitted  light.  It 
is  observed  by  polarized  or  parallel  transmitted  light. 

The  axes  of  absorption  coincide  generally  with  the 
axes  of  elasticity,  therefore  with  the  crystallographic 
axes  in  the  tetragonal,  hexagonal,  orthorhombic,  and  the 
b  axis  of  the  monoclinic  systems. 

Sections  perpendicular  to  the  optic  axis  can  not  show 


METHODS  OF   MINERAL  DETERMINATION  67 

differences  in  color  since  in  this  direction  the  absorp- 
tion must  be  equal  in  all  directions. 

Uniaxial  minerals  are  said  to  be  dichroic,  showing 
two  different  colors,  produced  by  the  rays  which  vibrate 
parallel  to  the  direction  of  the  vertical  axis  and  parallel 
to  the  plane  of  the  basal  axes. 

Biaxial  minerals  are  said  to  be  trichroic,  as  there 
are  theoretically  three  differences  in  color,  corresponding 
to  the  directions  of  the  three  axes  of  elasticity.  Ple- 
ochroism  exists  practically  only'in  colored  minerals. 

Pleochroism  may  be  tested  as  follows:  If  a  mineral 
is  pleochroic,  a  change  in  color  will  be  observed  upon 
rotating  the  stage.  This  may  appear  as  an  actual  change 
in  color  or  as  a  change  in  shade  of  the  same  color.  In 
case  it  is  almost  indistinguishable,  it  is  best  to  make 
the  test  with  the  condensing  lens  in  position  immediately 
under  the  section. 

An  absorption  formula  is  an  expression  of  these  dif- 
ferent amounts  of  absorption  in  any  mineral.  Thus 
a  >  b  indicates  that  absorption  is  greater  when  the  ether 
vibration  of  the  polarized  ray  is  parallel  to  the  crystal- 
lographic  axis  a  than  when  parallel  to  b. 

A  pleochroic  formula  expresses  the  colors  that  a  min- 
eral presents  in  polarized  light  vibrating  parallel  to  each 
of  its  axes  of  ether  vibration. 

For  magnesium  tourmaline  the  pleochroic  formula  is 
Z  •=  Dark  yellowish  brown. 
X  =  Pale  yellow. 

In  general,  amphiboles  show  pleochroism,  and  pyrox- 
enes do  not. 


68  OPTICAL  MINERALOGY  AND  PETROGRAPHY 


CHAPTER  5. 
Description  of  Important  Rock-making  Minerals. 

INTRODUCTION. 

The  present-day  classification  of  minerals  is  pri- 
marily a  chemical  one,  as  minerals  are  arranged  accord- 
ing to  the  acid  radical.  In  any  chemical  division,  how- 
ever, minerals  of  similar  chemical  composition,  if  related 
crystallographically,  are  placed  in  the  same  group,  as 
for  instance  the  six  members  of  the  calcite  group. 

About  a  thousand  kinds  of  minerals  are  known,  of 
which  most  are  rare  or  found  only  in  a  few  localities. 

Rogers  has  compiled  the  following  information,  which 
is  of  interest  in  a  discussion  of  the  derivation  of  min- 
eral names : 

The  following  minerals  were  named  in  honor  of  prom- 
inent scientists :  biotite  (Biot,  French  physicist),  brucite 
(Bruce,  an  early  American  mineralogist) ,  dolomite  (Dol- 
omieu,  French  geologist),  goethite  (Goethe,  the  German 
poet),  millerite  (Miller,  English  crystallographer) , 
scheelite  (Scheele,  Swedish  chemist),  smithsonite 
(Smithson,  founder  of  the  Smithsonian  Institution),  wol- 
lastonite  (Wollaston,  English  chemist) . 

The  following  minerals  were  named  from  prominent 
geographical  localities:  andalusite  (Andalusia,  a  prov- 
ince of  Spain),  aragonite  (Aragon,  ancient  kingdom  in 
Spain),  anglesite  (Anglesea,  in  Wales),  bauxite  (Beaux, 
in  France),  ilmenite  (Ilmen  mountains,  in  the  Urals), 


DESCRIPTION  OF  ROCK-MAKING  MINERALS  69 

labradorite  (Labrador),  muscovite  (Moscow,  in  Russia), 
strontianite  (Strontian,  in  Scotland). 

The  following  minerals  were  derived  from  the  Latin 
and  Greek  names  for  colors:  albite  (white),  azurite 
(blue),  cyanite  (blue),  celestite  (sky-blue),  chlorite 
(green),  erythrite  (red),  hematite  (blood),  leucite 
(white),  rhodonite  (rose-red),  rutile  (reddish). 

The  following  minerals  were  named  directly  from 
their  chemical  composition:  argentite,  arsenopyrite, 
barite,  calcite,  chromite,  cobaltite,  cuprite,  magnesite, 
molybdenite,  sodalite,  stannite,  zincite. 

At  one  time  there  existed  a  binomial  nomenclature 
for  minerals,  as  exist  at  the  present  time  for  animals 
and  plants.  Thus,  barite  was  known  as  Baralus  ponder- 
osus,  and  celestite  was  known  as  Baralus  prismaticus. 

The  following  minerals  are  discussed  according  to  the 
crystallographic  system  in  which  they  occur,  isotropic 
minerals  being  considered  first. 

ISOTROPIC  MINERALS. 

AMORPHOUS. 

OPAL. 
. .   Composition:    SiO,.    nH20. 

Criteria  for  determination  in  thin  section : 

Form:  No  crystal  form,  but  sometimes  concretion- 
ary, banded  or  with  spherulitic  structure. 

Optical  Properties:  n  =  1.45.  Relief  so  low  that  the 
mineral  may  be  mistaken  for  a  hole  in  the  section  filled 
with  balsam.  Feeble  negative  double  refraction  at  times. 
Colorless  patches  or  veins.  Fragments  are  dark  and 
irregular  between  crossed  nicols. 

Occurrence:  As  a  secondary  mineral  in  cavities  and 
seams  in  igneous  rocks;  as  sinter  around  hot  springs 
and  geysers  (Yellowstone  Park)  ;  as  a  constituent  of 


70  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

diatomaceous    earth.    Diatoms    and    radiolaria    secrete 
casts  of  opal  silica. 

Uses:  As  gems.  Precious  opals  are  found  in  New 
South  Wales  and  in  Hungary.  Fire  opal  is  found  in 
Mexico. 

ISOMETRIC 
PYRITE. 

Composition :    FeS2. 

Criteria  for  determination  in  thin  section : 

Form:  Cubes,  pentagonal  dodecahedrons  or  combi- 
nations of  these.  Sometimes  in  irregular  grains. 

Optical  Properties:  Opaque.  In  reflected  light,  pale 
brass-yellow  color  with  strong  metallic  luster. 

Alteration :  Alters  readily  to  limonite  by  oxidation 
and  hydration. 

Occurrence.  As  a  vein  mineral  associated  with  other 
sulphides.  As  an  original  and  secondary  mineral  in 
igneous  and  sedimentary  rocks. 

Uses:  Used  in  the  manufacture  of  sulphuric  acid. 
In  association  with  chalcopyrite  as  a  low  grade  copper 
ore.  It  is  often  gold-bearing. 

PYRRHOTITE. 

Composition:   Fe6S7  to  Fe^S^. 

Criteria  for  determination  in  thin  section : 

Form:  Practically  always  in  irregular  masses  and 
not  in  crystals.  Cleavage  usually  not  visible  microscop- 
ically. 

Optical  Properties.  Opaque.  Color  between  bronze 
yellow  and  copper  red.  Luster  metallic. 

Distinctions :  Distinguished  from  pyrite  by  its  usual 
association  in  irregular  masses  and  by  its  bronze  yellow 
color  in  incident  light. 

Occurrence:  In  basic  igneous  rocks;  as  a  vein  min- 
eral ;  in  crystalline  limestones. 


DESCRIPTION  OF  ROCK-MAKING  MINERALS  71 

Uses :  The  nickeliferous  pyrrhotite  of  Sudbury, 
Ontario,  is  an  important  ore  of  nickel. 

MAGNETITE. 

Composition :    Fe;!04. 

Criteria  for  determination  in  thin  section: 

Form :  Octahedrons  and  dodecahedrons.  Also  gran- 
ular. Cleavage  indistinct.  Twinning  common  after  0. 

Optical  Properties:  Opaque.  Bluish  black  by 
reflected  light,  with  a  strong  metallic  luster.  Index  of 
refraction  high. 

Alteration :    Alters  to  hematite,  limonite  and  siderite. 

Occurrence:  A  common  and  widely  distributed 
accessory  mineral  of  igneous  rocks;  magmatic  segrega- 
tion in  ore  deposits,  as  in  Scandinavia ;  as  a  contact  min- 
eral between  limestones  and  igneous  rocks;  in  lenses,  in 
gneisses  and  schists. 

Uses:  Important  ore  of  iron,  especially  in  New 
York,  New  Jersey  and  Pennsylvania. 

SPINEL. 

Composition :   MgAl,04. 

Criteria  for  determination  in  thin  section : 

Form :  In  grains  or  octahedral  crystals,  never 
decomposed  in  rocks. 

Optical  Properties:  Strictly  isotropic.  Index  of 
refraction  high.  Color  usually  the  lighter  shades  of  red, 
blue-green,  yellow,  brown.  The  most  common  colors  are 
green  (in  pleonaste,  iron-bearing)  and  coffee-brown  (ir. 
picotite,  chrome-bearing) . 

Differentiation:  Distinguished  from  garnets  by  its 
octahedral  form,  by  the  more  common  green  color,  by 
its  undecomposed  condition  and  by  its  slightly  lower 
relief. 

Occurrence:     As   a   contact   mineral,    in    crystalline 


72  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

limestones  and  schists.    As  an  accessory  mineral,  in  igne- 
ous rocks.    In  the  gem-bearing  gravels  of  Ceylon. 
Uses :     Ruby-spinel  is  used  as  a  gem. 

GARNET. 

Composition:  R",R'",  (Si04)3  R"  is  Ca,  Mg,  Fe  or 
Mn.  R'"  is  Al,  or  Fe. 

Criteria  for  determination  in  thin  sections : 

Form:  Irregular  grains  or  in  simple  crystals  as 
dodecahedrons.  Zonal  structure  frequent.  No  cleavage. 
Irregular  fracture. 

Optical  Properties:  Normally  isotropic,  sometimes 
showing  anomalous  double  refraction,  due  possibly  to 
internal  strain. 

Colorless  or  nearly  so,  to  yellowish  or  reddish. 

Index  of  refraction :    n  =  1.746  to  1.814. 
Relief  high  and  surface  rough. 

Alteration :  Usually  fresh.  May  be  found  altered  to 
chlorite. 

Differentiation:     From  spinel,  see  under  the  latter. 

Occurrence:  In  schists  and  gneisses,  granites,  peg- 
matites, peridotites,  nepheline  and  leucite-bearing  lavas, 
in  crystalline  limestones  developed  at  the  contact,  in 
sands. 

Uses:  As  an  abrasive,  particularly  for  finishing 
woodwork  and  leather.  Also  as  a  semiprecious  gem. 

LEUCITE. 

Composition :     KAlSi2O(i. 
Criteria  for  determination  in  thin  section : 
Form:    Grains,  or  well  defined,  embedded  crystals 
very  near  the  trapezohedron  or  tetragonal  trisoctahe- 
dron.    Cross  sections  round  or  eight-sided.    Vary  greatly 
in  size.     Fine  striations  due  to  twinning  common.     No 
cleavage,  though  fracture  may  be  noticed. 


DESCRIPTION  OF  ROCK-MAKING  MINERALS  73 

Optical  Properties: 

Colorless.    Refringence  low.    Relief  absent.    Sur- 
face smooth,    n  =  1.50. 

Birefringence  weak  but  distinct.     Colors  of  first 

order  (0.001). 

Inclusions :  Symmetrically  or  zonally  arranged,  con- 
sisting of  the  older  secretions  associated  with  leucite, 
magnetite,  apatite,  augite. 

Alteration:  Alters  readily  to  zeolites,  or  mixtures 
of  albite  and  sericite,  or  orthoclase  and  sericite. 

Occurrence :  Rare  in  the  United  States,  but  common 
in  Italy  in  basic  lavas,  substituting  the  feldspars. 

Differentiation :  Distinguished  from  all  minerals 
except  analcite  and  sodalite  by  its  low  refringence,  crys- 
tal form  and  twinning,  very  weak  birefringence.  From 
leucite  and  sodalite,  by  its  higher  refringence. 

SODALITE. 

Composition:    3  NaAlSiO4.    NaCl. 

Criteria  for  determination  in  thin  section : 

Form:  In  concentric  nodules.  Usually  in  dissem- 
inated or  massive  form  without  crystal  faces.  Crystals 
if  present  are  dodecahedrons.  Fracture  conchoidal. 
Cleavage  dodecahedral,  generally  invisible  in  thin 
section. 

Optical  Properties.  Isotropic.  May  be  weakly  bire- 
fringent  around  inclusions. 

Relief  absent.    Surface  smooth,   n  =  1.485. 
Color :     Colorless,  pink,  yellow,  blue. 

Alteration:  Common  to  fibrous  mass  of  zeolites  or 
to  aggregates  of  micaceous  minerals,  often  accompanied 
by  the  formation  of  limonite  and  calcite. 

Occurrence:  In  eruptive  rocks  rich  in  soda,  such  as 
nepheline  syenites. 

Differentiation:     From    nephelite    by    its   isotropic 


74  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

character.    From  other  isotropic  minerals,  by  a  very  low 
refractive  index. 

FLUORITE. 

Composition :     CaF2. 

Criteria  for  determination  in  thin  section : 

Form:     Crystals  cubical,   octahedral  and   dodecahe- 

dral.     Cleavage,  perfect  octahedral,  appearing  often  in 

section  as  triangular  cracks. 

Optical  Properties:     Isotropic. 

n  —  1.434.    On  account  of  the  low  index  of  refrac- 
tion the  negative  relief  is  marked. 

Abnormal  birefringence  may  show,  due  to  internal 
tension. 

Color  is  due  to  inclusions  of  hydrocarbons.     Some 
crystals  appear  green  by  transmitted  light  and  blue 
by  reflected  light.    Color  not  uniformly  distributed. 
Occurrence:     As     a     very     common     vein    mineral 

together  with  calcite,  barite,  sphalerite,  and  galena.    In 

limestones.    Kentucky  and  Illinois  are  chief  sources. 
Uses :     As  a  flux  in  iron-smelting  and  foundry  work. 

Also    for    the    manufacture    of    hydrofluoric    acid  and 

enamels. 


DESCRIPTION  OF  ROCK-MAKING  MINERALS  75 


CHAPTER  6. 
Description  of  Minerals    (Continued). 

ANISOTROPIC  MINERALS.  —  UNIAXIAL. 
TETRAGONAL. 

RUTILE. 

Composition :     Ti02. 

Criteria  for  determination  in  thin  section : 
Form:  Embedded  grains,  acicular  inclusions,  mas- 
sive or  in  crystals,  which  are  sharp,  elongated  and  pris- 
matic, or  in  net-shaped  groups.  Twinning  lamellae  com- 
mon in  basal  sections.  Prismatic  cleavage  not  observed. 
Elongation  parallel  to  c. 

Optical  Properties:     Uniaxial  and  positive. 

Refringence  very  high.  Relief  high  and  surface 
rough,  n  =  2.903  and  2.616. 

Birefringence  extreme.  Interference  colors  very 
high,  hence  may  not  be  noticed  when  mineral  is 
strongly  colored  (0.287). 

Extinction  parallel  to  prisms. 
Color :     Red,  brownish  red  to  black. 
Pleochroism  usually  not  noticeable.     X  is  yellow- 
ish, Z  is  brownish  yellow  to  yellowish  green. 
Alteration :     Quite  stable.    May  alter  to  ilmenite. 
Occurrence:     More  widely  distributed  as  a  micro- 
scopic mineral  than  as  one  of  megascopic  size.    Occurs  in 
igneous  and  metamorphic  rocks  and  in  veins.    As  a  sec- 
ondary mineral  in  clays.    Virginia  is  source. 


76  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

Uses:  As  a  source  of  ferro-titanium  and  as  a  col- 
oring matter  for  porcelain. 

ZIRCON. 

Composition :     ZrSiO4. 
Criteria  for  determination  in  thin  section : 
Form:   "Small,  short  prismatic  crystals  usually  elon- 
gated parallel  to  c.    Always  crystallized. 

Optical  Properties:     Uniaxial  and  positive. 

Refringence  very  high  and  surface  rough,  n  = 
1.983  and  1.93. 

Birefringence  very  strong  (0.053).  Interference 
colors  of  fourth  order,  minute  crystals  showing  bril- 
liant colors. 

Color :     Colorless  to  pale  gray  or  brown. 
Pleochroism    usually    not    noticeable,    but    when 
observed  little  absorption  takes  place  parallel  to  c. 
Extinction:     Parallel  to  c. 

Interference  figure  in  basal  section  shows  several 
rings  in  addition  to  dark  cross. 
Alterations :     Rare. 

Differentiations:  From  apatite,  by  much  higher 
relief  and  stronger  double  refraction.  From  cassiterite, 
by  much  weaker  double  refraction,  and  by  mode  of  occur- 
rence. 

Occurrence:  As  an  accessory  mineral  of  igneous 
rocks,  especially  the  more  acid  varieties.  In  sands  and 
gravels. 

WERNERITE   (SCAPOLITE  GROUP). 

Composition:    mCa4Al6O2ri.     +  nNa4Al3Si9O,4Cl. 

Criteria  for  determination  in  thin  section: 

Form:  Crystals  rough,  coarse  and  large,  in  cleav- 
able,  columnar  and  massive  forms.  Cleavage  distinct, 
parallel  to  square  prism.  Elongation  parallel  to  a. 

Optical  Properties :    Uniaxial  and  negative. 


DESCRIPTION  OF  ROCK-MAKING  MINERALS  77 

Refringence  considerable,  n  =  1.583  and  1.543. 
Relief  not  marked  and  about  the  same  as  quartz. 

Birefringence  rather  strong  (.03  to  .018).  Inter- 
ference colors  of  the  second  order  more  brilliant  than 
those  of  most  of  the  colored  minerals. 

Interference  figures  distinctly  uniaxial. 

Extinction  parallel  in  longitudinal  sections. 

Colorless. 

Alteration :     Alters  to  kaolinite,  muscovite,  etc. 
Differentiation:     From    feldspars,    by    absence    of 
twinning. 

From  quartz,  by  cleavage,  higher  order  of  inter- 
ference colors  and  optical  character.  Quartz  is 
positive. 

From  apatite,  by  lower  index  of  refraction,  cleav- 
age and  higher  order  interference  colors. 
Occurrence:     Found  in  gneisses,  crystalline  schists, 
and  limestones. 

HEXAGONAL. 

HEMATITE. 
Composition :     Fe20,. 
Criteria  for  determination  in  thin  section: 
Form:     Irregular  scales,  minute  grains  and  earthy. 
No  cleavage. 

Optical  Properties :     Uniaxial  and  negative. 

Refringence  very  high,    n  —  3.042  and  2.797. 

Birefringence  very  strong  (0.245). 

Opaque.  By  reflected  light,  black  with  tinge  of  red 
and  a  metallic  luster,  or  red  without  luster. 

Pleochroism  absent,  or  slight.  X  is  yellowish  red 
and  Z  is  brownish  red. 

Alteration :     Common  by  hydration  to  limonite. 
Occurrence:     Very  widely  disseminated.     As  micro- 
scopic inclusions  and  as  a  common  alteration  product  in 


78  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

all  rocks.     As  a  commercial  iron  ore  from  the  Lake 
Superior  district. 

ILMENITE. 

Composition :   FeTiO,,. 

Criteria  for  determination  in  thin  section : 

Form:  Irregular  masses,  without  crystallographic 
outline,  or  rhombohedral  crystals. 

Optical  Properties : 

Opaque.     Rarely  translucent,  and  dark  brown  in 

very  thin  sections.     Sometimes  brownish  in  reflected 

light,  with  metallic  luster. 

Alteration:  To  leucoxene,  which  is  believed  to  be  a 
variety  of  titanite.  This  alteration  often  develops  along 
definite  rhombohedral  directions. 

Differentiation:  From  magnetite,  by  occurring  in 
irregular  masses  and  by  a  whitish  strongly  refracting 
decomposition  product. 

Occurrence:  A  common  though  sparsely  distributed 
accessory  mineral  in  igneous  rocks  and  as  a  magmatic 
segregation  in  igneous  rocks. 

CORUNDUM. 
Composition :     A1203. 

Criteria  for  determination  in  thin  section : 
Form :     Prisms,    grains    or    plates.    Rhombohedral 
cleavage  may  show. 

Optical  Properties :     Uniaxial  and  negative. 

Colorless  or  with  patches  or  zones  of  blue. 

Refringence  very  high  and  surface  rough,     n  — 
1.7676  and  1.7594. 

Birefringence  weak  like  quartz    (0.082).     Inter- 
ference colors  middle  of  the  first  order,  yellow  to  blue. 

Interference  figure  of  basal  section  shows  indis- 
tinct cross. 


DESCRIPTION  OF  ROCK-MAKING  MINERALS  79 

Pleochroism  marked  when  color  is  deep.    Z  is  blue ; 

X  is  green. 

X  axis  coincides  with  crystallographic  a. 

Occurrence :  In  crystalline  metamorphic  rocks,  such 
as  marble,  gneisses,  mica  and  chlorite  schists,  in  perido- 
tites,  in  sands  and  gravels. 

Uses :  Ruby,  the  red  transparent  variety,  is  valuable 
as  a  gem.  Sapphire,  which  is  likewise  valued  as  a  gem, 
is  the  blue  transparent  variety.  Burma  furnished  the 
best  rubies  and  Ceylon  the  best  sapphires.  It  is  also 
used  as  an  abrasive. 

QUARTZ. 

Composition :     Si02, 

Criteria  for  determination  in  thin  section : 
Form:  Crystals  usually  prismatic,  terminated  by 
rhombohedrons.  Allotriomorphic  in  granitoid  rocks, 
rounded  grains  in  clastic  rocks.  Rarely  in  distinct  crys- 
tals in  any  rocks.  May  be  mutually  interpenetrated  by 
an  acid  feldspar.  Cleavage  nearly  always  absent  or 
difficult. 

Optical  Properties:     Uniaxial  and  positive. 

Colorless.    By  reflected  light  it  may  appear  cloudy 
if  it  contains  many  inclusions. 

Refringence  low.     No  relief  and  smooth  surface. 
n  =  1.553  and  1.554. 

Birefringence  weak  with   interference  colors  of 
white  or  yellow  in  the  middle  of  the  first  order  (0.009) . 
Pleochroism  absent. 

Extinction  takes  place,  but  is  not  distinctive,  due 
to  the  absence  of  cleavage  or  crystallographic  outline. 
Interference  figure  of  a  basal  section  shows  a  dark 
cross  without  any  rings. 

Alteration :  Does  not  alter,  so  that  the  fresh  appear- 
ance of  the  mineral  is  an  important  aid  in  identification. 


80  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

Inclusions:  Minute  fluid  or  gas  inclusions  common 
in  granitoid  rocks.  Not  so  abundant  in  porphyritic 
rocks. 

Occurrence:  One  of  the  most  abundant  minerals 
found  in  nature.  It  occurs  in  sedimentary,  acid  igneous, 
metamorphic  rocks  and  veins. 

Differentiation:  From  sanadine,  by  uniaxial  and 
positive  character. 

From  nephelite,  by  absence  of  hexagonal  outline, 

stronger  double  refraction,  and  fresh  undecomposed 

appearance. 

Uses:  For  ornamental  purposes,  for  optical  instru- 
ments, for  glass-making,  for  pottery  and  porcelain,  and 
as  an  abrasive. 

CALCITE. 

Composition :   CaCOy. 
Criteria,  for  determination  in  thin  section : 
Form:     Grains  and  aggregates.     May  be  fibrous  or 
oolitic.    Never  in  crystals  in  rocks.    Polysynthetic  twin- 
ning common,  probably  due  in  part  to  the  grinding  of 
the  section.    Shows  in  crossed  nicols  as  a  series  of  light 
and  dark  bands.     Cleavage  parallel  to  R  appearing  as 
many  cracks. 

Optical  Properties:     Uniaxial  and  negative. 

Colorless  when  pure,  but  may  appear  colored  by 
transmitted  light,  due  to  organic  pigments. 

Refringence  low.    Relief  not  marked  and  surface 
smooth,    n  =  1.658  and  1.486. 

Birefringence  very  strong,  with  pale,  iridescent 
interference  colors  of  the  fourth  order  (0.172). 

Extinction  parallel  to  cleavage  cracks  when  they 
appear. 

Pleochroism.     No  change  of  color  observed,  but 
absorption  can  be  noted  if  section  is  not  too  thin. 


DESCRIPTION  OF  ROCK-MAKING  MINERALS  81 

Interference  figure  of  basal  section  shows  distinct 
cross  and  rings. 
Inclusions  of  fluid  frequent. 

Differentiation:  From  other  carbonates,  difficult 
except  by  microchemical  tests. 

Occurrence:  Abundant  in  sedimentary  limestones 
and  as  a  decomposition  product  in  igneous  rocks.  Vein 
mineral  often  associated  with  ores  as  gangue.  As  trav- 
ertine and  cave  deposits. 

DOLOMITE. 

Composition:    (Ca.Mg)  CO3. 
Criteria  for  determination  in  thin  section : 
Form:     In  rocks  chiefly  as  crystals,  usually  unit  R, 
with  a  tendency  to  curved  surfaces.    As  dense  homogene- 
ous   aggregates    showing   tendency    toward    crystalline 
boundaries. 

Optical  Properties :     Uniaxial  and  negative. 

Similar  to  calcite,  from  which  it  may  be  differen- 
tiated by  slightly  higher  relief  (n  =  1.682  and  1.503) 
and  by  tendency  toward  crystalline  boundaries. 
Occurrence:     As  the  essential  constituent  of  dolo- 
mitic  limestones,  as  a  vein  mineral,  and  as  a  secondary 
mineral  in  cavities  in  limestone. 

Uses :  As  limestones  for  building  and  ornamental 
purposes.  Also  for  furnace  linings. 

SIDERITE. 

Composition :    FeC03. 

Criteria  for  determination  in  thin  section : 

Similar  to  calcite  in  form  and  optical  properties. 

Absorption  often  distinct. 

Alteration :  Changes  readily  on  exposure  to  limonite 
and  hematite. 

Differentiation:  From  calcite,  by  common  associa- 
tion with  limonite. 


82  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

From  dolomite  and  magnesite,  by  common  poly- 
synthetic  twinning.  It  is  the  only  mineral  of  the 
Calcite  group  with  both  indices  of  refraction  higher 
than  that  of  balsam  except  the  rarer  smithsonite  and 
rhodocrosite. 

Occurrence :     In  limestone,  clay  iron-stone,  clay  slate, 
gneiss.     Also  in  veins  with  metallic  ores. 

APATITE. 

Composition:   Ca,(Cl.F)   (P04)3. 
Criteria  for  determination  in  thin  section. 
Form :     Minute,  slender,  hexagonal  prisms,  with  reg- 
ular hexagonal  boundaries.     Grains.     Clusters  of  crys- 
tals.    Elongation     parallel     to     a.     Cleavage     seldom 
observed. 

Optical  Properties :     Uniaxial  and  negative. 

Colorless  usually  in  thin  section.  Sometimes  gray, 
blue  or  brown,  the  color  being  irregularly  distributed, 
due  perhaps  to  microscopic  inclusions. 

Refringence  moderate.  Relief  more  marked  than 
of  the  associated  colorless  minerals,  n  —  1.638  and 
1.634.  < 

Birefringence:     Weak,   with   interference   colors 
grayish  blue  or  white,  of  the  lower  first  order  (0.004) . 
Extinction  parallel  to  c  axis. 
Interference  figure  shows  a  cross  without  rings. 
Pleochroism  absent  for  white  crystals.     Colored 
varieties  weakly  pleochroic. 
Alterations :     Mineral  always  appears  fresh. 
Differentiation:     From    nephelite,   by   occurring   in 
smaller  and  longer  crystals,  and  invariably  fresher  in 
appearance. 

From  zircon,  see  under  the  latter  mineral. 
From  feldspars,  when  granular,  by  higher  relief 
and  the  uniaxial  interference  figure. 


DESCRIPTION  OF  ROCK-MAKING  MINERALS  83 

From  quartz,  in  having  a  higher  relief,  weaker 

birefringence,  and  a  negative  sign. 

Occurrence :  Widely  distributed  as  an  accessory  con- 
stituent of  igneous  rocks  and  in  crystalline  schists.  With 
metamorphic  limestones.  As  a  vein  mineral  in  gabbro 
and  in  pegmatites. 

Uses:     As  a  source  of  phosphates  for  fertilizers. 

NEPHELITE. 

Composition:     NaAlSiO4. 
Criteria  for  determination  in  thin  section : 
Form:     Crystals  thick,  six-sided  prisms  with  base 
prominent.    Massive  and  in  embedded  grains.    Cleavage 
imperfect,  parallel  to  the  prism  of  the  first  order  and 
the  base,  better  in  partially  altered  sections. 
Optical  Properties :     Uniaxial  and  negative. 
Colorless  in  thin  section. 

Refringence  low.  Relief  absent,  n  =  1.546  and 
1.542. 

Birefringence  very  weak  (0.005).  Interference 
colors  grayish  white  of  the  lower  first  order,  a  little 
lower  than  the  feldspar  colors. 

Extinction  parallel  to  cleavage  lines  when  they 
appear. 

Pleochroism  absent. 

Interference  figure  is  a  broad  cross  without  rings. 
Inclusions :     Microscopic  needles  of  augite,  also  fluid 
and  gas  generally  in  zones. 

Alteration :  Readily  to  fibrous  zeolites  with  stronger 
birefringence. 

Differentiation:  From  quartz,  by  weaker  birefrin- 
gence, better  hexagonal  outline,  and  negative  sign. 


84  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

From  feldspars,  by  uniaxial  character  and  absence 

of  twinning. 

Occurrence:  In  nephelite  syenites,  phonolites,  and 
rare  soda-rich  rocks.  It  is  never  associated  with  quartz, 
but  often  with  orthoclase. 

TOURMALINE. 

Composition:  R0Si05?  R  chiefly  Al,  K,  Fe,  Ca,  Mn, 
Mg,  Li. 

Criteria  for  determination  in  thin  section : 
Form:     Columnar  crystals,  bunched  or  in  radiating 
aggregates.  Irregular  cracks  may  appear,  but  no  cleavage 
is  seen.    Cross  section  shows  trigonal  outline  parallel  to 
base. 

Optical  Properties.     Uniaxial  and  negative. 

Color:  Varies,  with  grayish  blue,  brown,  and 
black  most  common.  Zonal  structure  may  be  shown 
by  differences  in  color. 

Refringence  medium.  Conspicuous  against  the 
colorless  rock  constituents.  Surface  rough,  n  — 
1.636. 

Birefringence  quite  strong  (0.02),  with  bright 
interference  colors  of  the  upper  first  or  lower  second 
order.  Often  masked  by  strong  absorption. 

Interference  figure  shows  a  sharp  cross  with  a 
few  rings. 

Extinction  parallel  to  the  c  axis. 

X  axis  is  parallel  to  the  c  axis. 

Pleochroism  distinct  even  in  light-colored  vari- 
eties, increasing  with  the  depth  of  the  color.  The 
greatest  absorption  takes  place  normal  to  the  direc- 
tion of  elongation  of  the  mineral.  Formula  for  Mg 
tourmaline  Z  is  pale  yellow.  X  is  colorless. 

Absorption  very  marked. 
Alteration  does  not  take  place  commonly. 


DESCRIPTION  OF  ROCK-MAKING  MINERALS  85 

Differentiation:  From  hornblende,  by  absence  of 
cleavage,  and  by  the  fact  that  the  greatest  absorption 
takes  place  at  right  angles  to  the  longitudinal  axis. 

Occurrence:  Widely  distributed  in  crystalline  schists 
and  gneisses,  in  crystalline  limestones  (New  Jersey),  in 
granite  pegmatites  and  veins  with  copper  minerals.  It 
is  a  common  product  of  contact  metamorphism. 

Uses :     Colored  tourmaline  is  used  as  a  gem. 


86  OPTICAL  MINERALOGY  AND  PETROGRAPHY 


CHAPTER  7. 
Description  of  Minerals    (Continued). 

ANISOTROPIC-BIAXIAL  MINERALS. 

ORTHORHOMBIC. 

ANDALUSITE. 

Composition :   Al2Si05. 
Criteria  for  determination  in  thin  section : 
Form :     Prismatic  crystals  always  more  or  less  elon- 
gated parallel  to  the  vertical  axis,  in  rough  or  embedded 
crystals.    Cleavage  may  show  parallel  to  almost  square 
prism. 

Optical  Properties:    Biaxial  and  negative. 
Color:     Colorless  to  reddish. 
Refringence  medium,    n  =  1.64  and  1.63. 
Birefringence  weak   (0.01).     Interference  colors, 
middle  of  the  first  order,  white  or  yellow. 

Interference  figure  shows  large  optic  angle. 
Extinction  parallel  to  c. 

Pleochroism  marked  only  in  colored  varieties, 
being  reddish  parallel  to  c,  which  is  the  direction  of 
elongation  or  cleavage.  Pleochroic  halos  often  sur- 
round inclusions. 

Inclusions:  Carbonaceous  matter  common,  distrib- 
uted through  the  crystal  in  some  geometrical  form  con- 
forming to  the  symmetry. 

Alteration:     Readily  to  colorless  mica. 
Differentiation:     From    diopside,    by    weaker  bire- 
fringence and  absence  of  extinction  angles. 


DESCRIPTION  OF  ROCK-MAKING  MINERALS  87 

Occurrence :     In  granitic  eruptive  rocks  and  in  meta- 
morphosed sedimentary  limestones. 

TOPAZ. 

Composition :  Al2F,SiO4. 
Criteria  for  determination  in  thin  section : 
Form :     Colorless  crystals  of  short  prismatic  habit. 
Cleavage  perfect  parallel  to  the  base. 
Optical  Properties:     Biaxial  and  positive. 

Refringence  medium,  about  the  same  as  calcite. 
n  =  1.617  and  1.607. 

Birefringence  weak  (0.01) ,  about  the  same  as  that 
of  quartz  with  interference  colors,  middle  of  the  first 
order  white  and  yellow. 

Interference  figure  shows  large  optic  angle. 
Extinction  parallel  to  cleavage. 
Z  axis  parallel  to  c. 

Alteration :     To  kaolin  or  muscovite  by  loss  of  F  and 
addition  of  water  and  alkalies. 

Differentiation :    From  quartz,  by  cleavage  and  biax- 
ial character. 

From  andalusite,  by  its  cleavage  and  its  smaller 
optic  angle. 

From  orthoclase,  by  its  higher  relief,  absence  or 
rarity  of  twinning  and  extinction  parallel  with  the 
cleavage. 

Occurrence:     In  contact  metamorphic  zones  and  in 
pegmatite,  associated  with  cassiterite,  fluorite,  tourma- 
line, beryl,  etc.    In  cavities  in  rhyolite. 
Uses :     Occasionally  as  a  gem. 

STAUROLITE. 

Composition:     FeAl5(OH)    (SiO0)2. 
Criteria  for  determination  in  thin  section : 
Form:     Short  prisms  twinned  at  90  or  60  degrees. 
Cleavage  variable,  both  prismatic  and  pinacoidal. 


88  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

Optical  Properties :     Biaxial  and  positive. 
Color :     Yellowish  to  brown. 
Refringence    rather    high    and    surface    rough. 
n  =  1.746  and  1.736. 

Birefringence  weak  (0.01)  with  interference  col- 
ors middle  of  first  order  white  to  yellow,  about  like 
quartz. 

Optic  angle  large.  Plane  of  optic  axis  is  parallel 
to  100. 

Pleochroism  distinct  but  not  strong,  showing 
the  darker  color  parallel  to  c,  the  direction  of  elon- 
gation (Z  is  golden  yellow,  Y  is  pale  yellow,  X  is  col- 
orless). 

Extinction  parallel  to  cleavage  or  crystal  outline. 
Inclusions  of  rutile,  tourmaline,  garnet  and  quartz 
occur,  the  latter  abundantly. 

Alteration :   To  a  green  mica  and  chlorite. 
Differentiation:     From  titanite,  by  the  fact  that  in 
convergent  light  the  optic  plane  is  shown  to  be  in  the 
longer  diagonal  of  the  cross  section. 

Occurrence :    In  mica  schists  and  phyllites  associated 
with  garnet,  cyanite  and  andalusite. 
SERPENTINE. 

Composition :     H4Mg3Si2O9. 
Criteria  for  determination  in  thin  section: 
Form:    Not   known   in   crystal   form.      Fibrous   or 
scaly  masses  with  elongation  parallel  to  c.     Prismatic 
cleavage  of  130  degrees  seldom  visible. 

Optical  Properties:     Biaxial  and  positive. 

Color  in  thin  section:  Pale  green,  yellow,  or 
colorless. 

Refringence  low,  about  the  same  as  Canada  bal- 
sam. No  relief,  and  smooth  surface,  n  =  1.54. 

Birefringence    rather    weak,    with    interference 


DESCRIPTION  OF  ROCK-MAKING  MINERALS  89 

colors  middle  of  the  first  order,  gray,  white  or  yellow. 
Between  crossed  nicols  the  aggregate  structure  is 
distinctly  seen.  Fine  fibrous  aggregates  may  appear 
isotropic  (0.013). 

Pleochroism  in  thick  sections  distinct.    Z  is  green 
or  yellow.    Y  and  X  are  greenish  yellow  to  colorless. 
Optic  plane  parallel  to  010.    Optic  angle  is  small. 
Extinction  parallel. 

Differentiation:  From  chlorite,  by  its  weaker  ple- 
ochroism. 

From  fibrous  amphiboles,  by  much  weaker  bire- 
fringence,    lower    relief    and     parallel     extinction. 
Fibrous  structure  and  color  indicative. 
Occurrence:     As   an   alteration   product  of  olivine, 
amphiboles,   pyroxenes.     The   essential  mineral   in  the 
metamorphic  rock  serpentine,  derived  from  peridotite. 
A  secondary  occurrence  in  veins. 

Uses :  As  an  ornamental  stone.  The  fibrous  variety 
forms  a  commercial  asbestos. 

THE  ORTHORHOMBIC  PYROXENES. 

ENSTATITE  AND  HYPERSTHENE. 

ENSTATITE. 

Composition :   MgSiO;i. 

Criteria  for  determination  in  thin  section : 

Form :  Distinct  crystals  rare,  prismatic.  Columnar 
or  fibrous  structure  parallel  to  c,  characteristic  of  allo- 
triomorphic  occurrences.  Usually  massive,  fibrous  or 
lamellar.  Prism  angle  nearly  90  degrees.  Twinning  not 
as  common  as  in  the  monoclinic  pyroxenes.  Prismatic 
cleavage  distinct. 

Optical  Properties :     Biaxial  and  positive. 

Color:     Colorless    in    thin    sections.      Bronzite, 

which  is  a  variety  of  enstatite  containing  ferrous  iron 


90      OPTICAL  MINERALOGY. AND  PETROGRAPHY 

in  place  of  some  of  the  magnesium,  is  colorless  or 
nearly  so,  and  shows  strong  pleochroism  with  X  a 
pale  yellow,  Y  a  brownish  yellow,  and  Z  a  bright 
green. 

Refringence  high  and  surface  rough,  about  the 
same  as  in  the  monoclinic  pyroxenes,  n  =  1.665  and 
1.656. 

Birefringence  weak  (0.009)  much  weaker  than 
the  monoclinic  pyroxenes.  Interference  colors  low 
of  first  order. 

Interference  figures  not  marked  on  account  of  the 
weak  double  refraction. 

Extinction  parallel  to  cleavages,  both  pinacoidal 
and  longitudinal  prismatic,  and  bisecting  angles  of 
intersecting  prismatic  cleavages. 

Axial  plane  parallel  to  brachypinacoid,  that  is, 
parallel  to  the  best  cleavage.  Axial  angles  large. 

Pleochroism  weak  or  absent. 

Alteration:  To  serpentine  by  ordinary  weathering. 
Also  to  uralite  (a  variety  of  hornblende) ,  but  much  less 
commonly  than  the  monoclinic  pyroxenes  do. 

Differentiation:  From  hypersthene,  by  the  optic 
sign  and  absence  of  distinct  color  and  pleochroism. 

From  the  monoclinic  pyroxenes,  by  parallel  extinc- 
tion on  vertical  sections,  and  lower  interference 
colors. 

Occurrence:  A  common  constituent  of  basic  igneous 
rocks  as  well  as  of  serpentine  derived  from  them.  Also 
found  in  crystalline  schists  and  in  many  meteorites. 
Bronzite  contains  about  10  per  cent  FeO  and  has  a  char- 
acteristic bronzy  luster  due  to  inclusions. 

HYPERSTHENE. 

Composition:    (Mg,  Fe)  SiO.,. 

Criteria  for  determination  in  thin  section : 


DESCRIPTION  OF  ROCK-MAKING  MINERALS  91 

Form:  Similar  to  enstatite.  More  often  massive  in 
lamellae.  Elongated  parallel  to  c. 

Optical  Properties:     Biaxial  and  negative. 
Color :  Brownish  to  greenish. 
Refringence  slightly  higher  than  enstatite,  due  to 
increase  in  percentage  of  iron. 

Birefringence  slightly  stronger  than  enstatite,  due 
to  increase  of  iron.  Weaker  than  monoclinic 
pyroxenes. 

Extinction  same  as  enstatite. 
Axial  plane  parallel  to  brachypinacoid,  i.e.,  parallel 
to  the  best  cleavage.     Optic  angle  about  X  becomes 
smaller  with  increase  in  iron  content. 

Pleochroism  distinct,  increasing  with  increase  in 
iron.  Z  is  bright  green,  Y  is  yellowish  brown,  X  is 
clear  red. 

Inclusions:  Gaseous,  liquid,  glassy.  Also  a  reddish 
brown  material  regularly  arranged,  which  gives  it  a 
peculiar  submetallic  bronze-like  luster.  They  are 
believed  to  be  inclusions  of  ilmenite,  either  primary  or 
produced  at  depth  under  pressure  by  circulating  waters 
acting  along  a  cleavage  or  parting  plane. 

Alteration:  To  a  variety  of  serpentine  called  bas- 
tite,  less  commonly  to  uralite,  occasionally  to  talc. 

Occurrence:  Important  constituent  with  plagioclase 
in  basic  igneous  rocks,  as  norites  and  gabbros.  Abundant 
in  andesites.  Found  in  meteorites. 

BASTITE. 

Bastite  is  a  variety  of  serpentine  to  which  the  ortho- 
rhombic  pyroxenes  poor  in  iron  alter  frequently  through 
the  ordinary  processes  of  weathering.  It  is  geometrically 
oriented  on  the  altered  pyroxene,  replacing  crystal  for 
crystal.  It  is  composed  of  fibers  often  traversed  by  irreg- 
ular cracks.  Cleavage  traces  of  the  two  minerals  coin- 


92  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

cide,  but  the  optical  properties  differ.  The  pyroxene  has 
a  cleavage  parallel  to  the  trace  of  the  optic  plane.  In 
bastite,  the  cleavage  is  perpendicular  to  the  trace  of  the 
optic  plane,  and  to  the  negative  acute  bisectrix.  This 
is  the  surest  distinction  between  them. 

Bastite  is  light  yellowish  or  greenish.  Refringence 
is  less  than  that  of  the  orthorhombic  pyroxenes  and  about 
the  same  as  Canada  balsam.  Birefringence  is  weak. 
Extinction  is  parallel  to  the  fibres.  Pleochroism  is  weak 
and  seen  only  in  thick  sections. 

OLIVINE    (CHRYSOLITE). 

Composition:    (Mg.  Fe)2  Si04. 
Criteria  for  determination  in  thin  section : 
Form :     Idiomorphic,  or  in  grains  or  granular  aggre- 
gates.    Also  massive.     Longitudinal  sections  more  or 
less  lath-shaped  with  pointed  ends.    Outlines  of  crystals 
often    corroded    or    rounded.      Interpenetration    twins 
occur.     Cleavage   distinct,   parallel   to   brachypinacoid, 
less  distinct  parallel  to  macropinacoid,  often  made  more 
visible  by  decomposition.     An  irregular  fracturing  is 
often  conspicuous,   especially  where  alteration  to  ser- 
pentine has  commenced.    Elongation  usually  parallel  to  c. 
Optical  Properties :     Biaxial  and  positive. 

Color:  Nearly  colorless,  becoming  reddish  with 
high  iron  content. 

Refringence  high.  Relief  marked  and  surface 
rough,  n  =  1.689  and  1.6535. 

Birefringence  very  strong,  with  interference  colors 
of  the  second  or  third  order,  higher  than  the  colors 
of  augite  (0.0359). 

Extinction  always  parallel  to  cleavage  cracks. 
Axial  plane  parallel  to  the  base,  that  is,  at  right 
angles  to  the  general  direction  of  elongation.    Axial 
angle  very  large. 


DESCRIPTION  OP  ROCK-MAKING  MINERALS  93 

Pleochroism  absent  except  in  reddish  varieties. 

Inclusions :  Magnetite,  spinel,  apatite,  common.  Also 
liquid  or  gas. 

Alteration:  Alters  readily.  Altered  forms  are  more 
frequently  observed  than  the  fresh.  Serpentine  is  the 
commonest  alteration  product,  with  frequently  a  separa- 
tion of  magnetite  or  hematite.  The  first  alteration  goes 
on  along  the  cleavage  and  fracture  cracks. 

It  is  easily  altered  by  atmospheric  weathering  to  car- 
bonates with  limonite  and  opal  or  quartz.  Calcite  may 
usually  be  distinguished  in  this  case.  In  contact  with  a 
feldspar  it  may  alter  to  an  amphibole  by  regional  meta- 
morphism.  The  amphibole  appears  as  a  zone  of  pale 
green  or  colorless  needles  between  the  olivine  and  the 
feldspar. 

Differentiation :  From  light  colored  monoclinic  pyrox- 
enes by  parallel  extinction,  by  poorer  and  unequal  cleav- 
ages and  stronger  birefringence.  Olivine  should  be  easily 
recognized  by  its  high  refringence,  a  shagreen  surface,  no 
color,  strong  birefringence,  and  a  large  optic  angle. 

Occurrence:  Especially  in  basic  igneous  rocks,  asso- 
ciated with  augite,  hypersthene,  plagioclase,  magnetite. 
An  essential  constituent  of  many  meteorites,  constituting 
the  stony  portion  of  the  mass. 

Uses:  The  transparent  variety  is  sometimes  used  as 
a  gem  under  the  name  peridot. 

TALC. 

Composition :    H,Mg;(  ( SiO:! )  4. 
Criteria  for  determination  in  thin  section : 
Form:    Colorless  plates,  elongated  like  rods.     More 
rarely  with  round  to  hexagonal  outline.    May  be  arranged 
in  rosettes.    Often  more  or  less  compact  foliated  masses. 

Cleavage  perfect  parallel  to  the  base  like  mica.    Elonga- 
tion parallel  to  c. 


94  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

Optical  Properties:     Biaxial  and  negative. 
Colorless  in  thin  section. 
Refringence  moderate,    n  —  1.589  and  1.539. 
Birefringence  strong,  with  interference  colors  of 
the  third  order,  like  muscovite  (0.05  to  0.035). 
Extinction  parallel  to  basal  cleavage  lines. 
Plane  of  optic  axes  parallel  to  100.     Optic  angles 
small. 

Differentiation:  From  muscovite,  by  its  small  optic 
angle.  From  sericite,  by  lower  refringence. 

Occurrence:  Most  abundantly  in  crystalline  schists, 
often  forming  rock  masses,  as  soapstone.  As  a  secondary 
mineral  in  basic  igneous  rocks,  altering  from  olivine,  en- 
statite,  tremolite. 

Uses:  For  soap,  talcum  powder,  French  chalk,  and 
in  the  manufacture  of  paper. 

NATROLITE. 

Composition:     Na2ALSi.,010.  2H2O. 
Criteria  for  determination  in  thin  section: 
Form :  Aggregates  of  colorless,  fibrous  crystals,  often 
in   interlacing   groups   or   divergent.      Prismatic    angle 
nearly  90  degrees.    Elongation  parallel  to  c.    Microscopic 
twinning  on  110. 

Optical  Properties:    Biaxial  and  positive. 

Refringence  very  low,  with  no  relief,     n  =  1.485 
and  1.473. 

Birefringence  weak,  though  slightly  stronger  than 
quartz. 

Interference  colors,  middle  of  the  first  order  yel- 
low, a  little  higher  than  quartz. 

Interference  figure  shows  dark  cross.    Optic  angle 
large. 

Plane  of  optic  axis  parallel  to  010. 

Axis  Z  parallel  to  c. 


DESCRIPTION  OF  ROCK-MAKING  MINERALS  95 

Extinction  parallel  to  fibers. 

Occurrence :  Never  found  as  a  primary  mineral.  As 
a  secondary  mineral  in  basic  igneous  rocks  filling  amyg- 
daloidal  cavities.  Common  alteration  product  of  sodalite, 
nephelite,  and  acid  plagioclase. 

PYROXENE  GROUP. 

Composition :  The  pyroxenes  are  metasilicates  of  cal- 
cium, magnesium,  iron,  or  more  complex  silicates,  often 
containing  two  or  more  bases,  both  bivalent  and  triva- 
lent.  They  are  closely  related  to  each  other  in  crystallo- 
graphic  and  physical  properties.  They  crystallize  in  the 
orthorhombic,  monoclinic  and  triclinic  systems. 
Criteria,  for  determination  in  thin  section : 
Form :  Fundamental  form  is  a  short  prism  with  inter- 
facial  angles  of  about  87  and  93  degrees.  Distinct  cleav- 
age occurs  parallel  to  both  prism  faces.  Twinning  if 
present  is  parallel  to  100.  Elongation  usually  parallel 
to  c. 

Optical  Properties :   Biaxial.    Most  species  positive. 

Color:  In  thin  section  usually  pale  to  colorless. 
Soda  pyroxenes  have  a  distinct  green  color. 

Refringence  high.  Relief  distinct  and  surface 
rough,  n  =  1.68  to  1.72. 

Birefringence  strong,  being  stronger  in  the  pale 
or  colorless  pyroxenes  with  interference  colors, 
bright  tints  of  the  second  order  (0.021  to  0.030). 

Extinction :  Maximum  angle  from  0  to  95  degrees, 
with  common  species  varying  between  30  and  54  de- 
grees. In  sections  showing  parallel  cleavage  lines, 
parallel  extinction  in  orthopinacoidal  sections,  and  in 
all  other  sections  an  extinction  angle  is  observed.  The 
maximum  extinction  angle  is  large  and  is  obtained 
only  when  the  section  of  the  crystal  is  parallel  to  the 
clinopinacoid. 


96  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

Optic  plane  is  parallel  to  010. 
Pleochroism  weak  or  absent  except  in  the  soda 
pyroxene. 

Alteration :   Alters  readily  to  amphiboles.    Described 
under  each  species. 

Differentiation:     Pyroxenes    may    be    distinguished 
from  amphiboles  by  the  following  criteria : 

Pyroxenes.  Amphiboles. 

Cleavage  angle  about  93  de-     Cleavage  angle  about   124 

grees.  degrees. 

Crystals  short  prismatic.         Crystals  long  prismatic: 
Color  usually  weak.  Color  marked. 

Pleochroism  weak.  Pleochroism  marked. 

Extinction  angles  0-95  de-     Extinction  angles  0-25  de- 
grees, grees. 
Most  species  positive.               Most  species  negative. 
Alter  to  amphiboles.                Alter   to   chlorite,   biotite, 

etc. 

MONOCLINIC  MINERALS. 
Monoclinic  Pyroxenes. 

Diopside. 
Diallage. 
Augite. 
Aegirite. 
DIOPSIDE. 

Composition:    Ca(Mg,  Fe)   (Si03)2. 
Criteria,  for  determination  in  thin  section : 
Form:     Long,  columnar  crystals  and  grains.     Often 
coarsely  lamellar.     Granular  masses.     Cleavage  always 
distinct  parallel  to  110  in  two  directions  nearly  at  right 
angles  to  each  other.    Parting  parallel  to  the  base,  yield- 
ing fine  twinning  lamellae  in  this  direction.    Elongation 
parallel  to  c. 


DESCRIPTION  OF  ROCK-MAKING  MINERALS  97 

Optical  Properties :     Biaxial  and  positive. 

Colorless  usually  in  thin  section ;  but  with  increase 
in  iron,  color  becomes  distinctly  greenish. 

Refringence  increases  with  increase  in  iron  con- 
tent,   n  =  1.7026  and  1.6727. 

Birefringence  decreases  with  increase  in  iron  con- 
tent (0.0299). 

Interference  colors  bright,  of  the  second  order. 
Interference  figure  is  an  axial  bar  with  concentric 
rings. 

Extinction  angle  from  20  to  30  degrees. 
Plane  of  the  optic  axes  parallel  to  010. 
Dispersion  weak. 

Inclusion:  Gaseous,  liquid,  or  glassy,  arranged  in 
zones. 

Alteration :  Most  commonly  to  serpentine  or  to  an 
aggregate  of  serpentine  and  chlorite,  often  with  calcite 
and  quartz.  Also  to  actinolite  or  hornblende,  alteration 
starting  around  the  periphery  or  along  cleavage  cracks. 
Differentiation:  From  augite,  by  less  dispersion, 
consequently  better  extinction  in  white  light. 

From  segirite  and  spodumene,  by  the  extinction 
angle  in  the  vertical  zone. 

From  hypersthene,  by  the  absence  of  pleochroism. 
From  orthorhombic  pyroxenes,  by  the  extinction 
angle  and  the  higher  order  of  colors. 
Occurrence:     In  crystalline  limestones  as  a  contact 
mineral  with  garnet.    In  many  igneous  rocks  as  granites, 
diorites,  syenites,  gabbros,  and  peridotite.    In  metamor- 
phic  rocks. 

DIALLAGE. 

A  variety  of  diopside  showing  well  developed  parting 
parallel  to  110,  generally  showing  a  fibrous  tex- 
ture parallel  to  c.  Color  usually  brown,  with  pleochroism 


98  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

as  follows:  Z  is  greenish,  Y  is  brownish  or  reddish 
brown,  X  is  greenish.  It  contains  inclusions  like  those 
of  hypersthene,  which  gives  it  a  bronze-like  luster. 

AUGITE. 

Composition:  mCaMg  (SiO;!)2.    n(Mg,  Fe)   (Al,Fe)L. 
Si06. 

Criteria  for  determination  in  thin  section : 
Form :     Crystals  short  thick  prisms  coarsely  lamellar, 
parallel  to  001  or  100.  Granular.  Twinning  common,  giv- 
ing  polysynthetic   lamellae   parallel   to    100.      Cleavage 
imperfect,  but  distinct  in  two  directions  parallel  to  110 
nearly  at  right  angles.    Elongation  parallel  to  c. 
Optical  Properties :     Biaxial  and  positive. 

Color  green,  greenish  black,  brown.  Rarely 
yellow. 

Refringence  high.  High  relief  and  rough  surface. 
n  =  1.733  and  1.712. 

Birefringence  rather  strong,  being  stronger  in  the 
pale  or  colorless  pyroxenes.  Interference  colors  are 
bright  tints  of  the  second  order  (0.021) . 

Interference  figures  distinct  on  account  of  the 
strong  birefringence.  Axial  angles  large. 

Optic  plane  parallel  to  the  clinopinacoid  (010). 
Extinction  angle:  Maximum  from  38  to  51 
degrees,  which  is  obtained  when  the  section  of  the 
crystal  is  parallel  to  the  clinopinacoid  (010),  varying 
from  these  angles  to  0  degrees  when  the  section  is 
parallel  to  the  orthopinacoid  (100). 

Pleochroism  usually  absent  or  weak  unless  rich  in 
iron,  in  which  case  Z  is  greenish,  Y  is  brownish  to  red- 
dish brown,  and  X  is  green. 

Inclusions:      Gaseous,    liquid    or   glassy,    sometimes 
arranged  in  zones. 


DESCRIPTION  OF  ROCK-MAKING  MINERALS  99 

Alteration:  Most  commonly  to  uralite,  a  variety 
of  hornblende,  either  crystal  for  crystal,  or  to  a  fibrous 
aggregate  of  uralite.  The  alteration  begins  around  the 
periphery  of  the  crystal  or  along  cleavage  cracks.  It 
may  alter  to  biotite  and  then  to  chlorite,  or  directly  to 
chlorite,  sometimes  forming  calcite,  quartz  or  epidote 
simultaneously. 

Differentiation :    From  diopside,  see  under  the  latter. 
From  segirite  and  spodumene  in  the  extinction 
angle  in  the  vertical  zone. 

From  amphiboles,  see  under  Pyroxene  group. 
From  epidote,  by  the  fact  that  the  plane  of  the 
optic  axis  is  parallel  to  the  longitudinal  axis  and  cleav- 
age cracks. 

Occurrence:  Abundant  in  igneous  rocks,  but  found 
also  in  metamorphic  rocks.  Occurs  in  some  stony 
meteorites. 

^GIRITE  (ACMITE). 
Composition:   Na  Fe  (SiO3)2. 
Criteria  for  determination  in  thin  section : 
Form:    In  crystal  form,  similar  to  augite,  although 
often  longer  or  acicular.    Cleavage  parallel  to  110,  more 
distinct  than  in  augite,  almost  at  right  angles.     Part- 
ing parallel  to  100.    Elongation  parallel  to  c. 
Optical  Properties :    Biaxial  and  negative. 

Color  in  thin  section  greenish  or  brownish. 
Refringence  high,  with  high  relief  and  rough  sur- 
face,   n  =  1.8126  and  1.762. 

Birefringence  quite  strong,  with  bright  tints  of 
the  second  order,  although  it  is  stronger  in  the  pale 
or  colorless  varieties  of  pyroxene  (0.0496). 

Interference  figures  distinct  on  account  of  strong 
birefringence.  Optic  angle  is  large. 

Optic  plane  parallel  to  the  clinopinacoid  (010). 


100  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

Extinction  angle:    About  5  degrees. 
Pleochroism  marked.     Z  is  yellowish  green,  Y  is 
olive  green,  X  is  dark  grass  green. 
Alteration :   To  analcite  and  to  the  iron  oxides. 
Differentiation :     From   the    amphiboles,    see    under 
pyroxene  group. 

From  other  monoclinic  pyroxenes,  by  the  very 
small  extinction  angle,  the  negative  sign,  the  stronger 
birefringence  and  marked  pleochroism. 
Occurrence :   In  pegmatite  veins,  in  soda-rich  igneous 
rocks,    as    nephelite,    syenites,    phonolites    and    soda- 
granites. 

AMPHIBOLE  GROUP. 

Composition:  The  minerals  of  the  amphibole  group 
are  orthorhombic,  monoclinic,  and  triclinic  silicates  of 
magnesium,  calcium  iron  or  sodium,  with  aluminum  or 
ferric  iron  in  some  cases. 

Criteria  for  determination  in  thin  section : 
Form:   Crystals  usually  prismatic,  elongated  parallel 
to  c,  possessing  very  marked  and  regular  prismatic  cleav- 
ages, varying  little  from  124  degrees  between  the  cleav- 
age faces.    Twinning  common  parallel  to  100. 

Optical  Properties:    Biaxial.     Most  species  negative. 
Color  in  thin  section  are  green,  brown,  blue,  yel- 
low or  colorless. 

Refringence  averages  less  than  the  pyroxenes, 
increasing  with  increase  of  iron.  Relief  distinct, 
n  =  1.621  to  1.642. 

Birefringence  quite  strong,  but  a  little  weaker 
than  in  the  pyroxenes.  Interference  colors  are  bright 
tints  of  the  second  order.  May  be  masked  by  strong 
absorption  (0.019  to  0.027). 

Extinction :    Maximum  angle  from  0  to  25  degrees. 


DESCRIPTION  OF  ROCK-MAKING  MINERALS  101 

In  the  monoclinic  amphiboles  the  axis  Z  makes  an 
angle  with  the  vertical  crystallographic  axis  c,  which 
varies  from  0  to  22  degrees,  except  in  uncommon 
species.  Elongation  therefore  positive. 

Pleochroism  distinct  and  intense  in  the  colored 
varieties. 

Absorption  very  marked,  being  greatest  in  the 
general  direction  of  the  cleavage  lines  in  the  longi- 
tudinal sections  (parallel  to  Z). 

Optic  plane  parallel  to  010  in  monoclinic 
amphiboles. 

Inclusions:  Iron  ores,  apatite,  etc. 
Alteration :  Alter  readily  to  chlorite,  biotite,  sericite, 
epidote,  calcite,  talc,  etc.,  the  process  being  gradual,  and 
usually  beginning  along  the  edges  and  the  cleavages  of 
the  amphibole  until  all  trace  of  the  original  mineral  is 
lost. 

Differentiation:    Amphiboles   may   be    distinguished 
from  the  pyroxenes  by  the  following  criteria : 

Amphiboles.  Pyroxenes. 

Cleavage   angle  about   124  Cleavage  angle    about    93 

degrees.  degrees. 
Crystals  long  prismatic.  Crystals  short  prismatic. 
Color  marked.  Color  usually  weak. 
Pleochroism  marked.  Pleochroism  weak. 
Extinction  angles,  0-25  de-  Extinction  angles,  0-95  de- 
grees, grees. 
Most  species  negative.  Most  species  positive. 
Alter   to    chlorite,    biotite,  Alter  to  amphiboles. 
etc. 

Occurrence:    In  all  classes  of  eruptive  rocks  and  in 

many  metamorphic  rocks.  Often  formed  by  alteration 
from  pyroxenes. 


102  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

Monoclinic  Amphiboles: 

Tremolite. 
Actinolite. 
Hornblende. 
Riebeckite. 

TREMOLITE. 

Composition :     Ca  Mg,  ( Si03 )  4. 
Criteria  for  determination  in  thin  section : 
Form :  Crystals  long-bladed  or  short  prismatic.  Often 
fibrous  or  acicular.     Perfect  prismatic  cleavage  at  an 
angle  of  about  124  degrees.    Cleavage  sometimes  distinct, 
parallel  to  010  and  100.    Transverse  fracture  frequent. 
Cleavage  more  perfect  than  in  pyroxenes. 
Optical  properties:   Biaxial  and  negative. 
Colorless  in  thin  section. 

Refringence  high,  with  distinct  relief,  but  not  as 
marked  as  in  the  pyroxenes,    n  =  1.634  and  1.6065. 

Birefringence  quite  strong,  but  a  little  weaker 
than  in  pyroxenes  (0.0275). 
Optic  plane  parallel  to  010. 

Maximum  extinction  angle  is  18  to  16  degrees  in 
vertical  zone. 

Dispersion  weak. 

Inclusions  of  carbonaceous  matter  and  biotite  in 
tremolite  of  metamorphic  rocks. 

Alteration:  To  talc,  beginning  along  cleavage  lines. 
Also  to  calcite. 

Differentiation :  From  hornblende,  by  light  color.  It 
has  the  lowest  index  of  refraction  found  in  monoclinic 
amphiboles. 

From  pyroxenes,  see  under  Amphibole  group. 
Occurrence :   In  schists,  contact  rocks,  and  veins. 
Uses:   Fibrous  varieties  sometimes  used  as  asbestos. 
As  jade,  sometimes  used  for  ornamental  purposes. 


DESCRIPTION  OP  ROCK-MAKING  MINERALS  103 

ACTING-LITE. 

Composition :    Ca  ( Mg,  Fe)  3  ( SiO;! )  4. 
Criteria  for  determination  in  thin  section: 
Form :   Similar  to  tremolite. 
Optical  Properties:    Biaxial  and  negative. 

Color  in  thin  section  pale  to  dark  green,  depending 
upon  the  percentage  of  iron. 

Refringence  and  birefringence  similar  to  tremo- 
lite (0.025). 

Maximum  extinction  angle  in  vertical  zone  is  15 
degrees. 

Dispersion  weak. 

Pleochroism  pronounced,  and  absorption  marked, 
being  greatest  in  the  general  direction  of  the  cleavage 
lines  in  longitudinal  sections.    Z  is  pale  to  dark  green, 
Y  is  greenish  yellow,  X  is  very  pale  yellow. 
Inclusions :   Similar  to  tremolite. 
Alteration :   To  chlorite,  epidote,  talc,  etc. 
Occurrence :  Same  as  tremolite,  with  which  it  is  often 
associated.     Uralite  is  the  name  given  to  the  amphibole 
to  which  the  pyroxenes  frequently  alter.    It  usually  cor- 
responds to  actinolite. 

HORNBLENDE. 

Composition:    mCa(Mg,   Fe)C)    (SiO,)4.     n(Al,   Fe) 
(P,OH)SiOs. 

Criteria  for  determination  in  thin  section : 
Form :  Prismatic  elongated,  parallel  to  the  vertical 
axis,  sometimes  fibrous.  Prismatic  cleavage  perfect, 
making  the  characteristic  angle  of  124  degrees.  Parting 
and  polysynthetic  twinning  are  sometimes  present,  par- 
allel to  100  or  001.  Cross  sections  may  be  acutely 
rhombic,  with  acute  angles  truncated,  hence  six-sided, 
whereas  the  pyroxenes  are  usually  eight-sided.  Longi- 
tudinal sections  lath-shaped.  Zonal  structure  occurs  fre- 


104  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

quently  in  the  brown  hornblende.     Twinning  frequently 
parallel  to  the  orthopinacoid. 

Optical  Properties:   Biaxial  and  negative. 

Colorless,  gray,  green,  greenish  blue,  brown  or 
black. 

Refringence  high,  with  distinct  relief,  n  =  1.653 
and  1.629. 

Birefringence  quite  strong,  being  strongest  in 
the  basaltic  hornblende.  In  common  hornblende, 
n=  (0.024).  In  basaltic  hornblende,  n=  (0.072). 
Interference  colors,  bright  tints  of  the  second  order, 
often  masked  by  strong  absorption. 

Maximum  extinction  angle  in  common  hornblende 

20  degrees,  in  basaltic  hornblende  from  1  to  2  degrees. 

Optic  plane  parallel  to  010,  in  which  face  Z  makes 

a  variable  angle  with  the  axis  c  in  the  obtuse  angle 

Beta. 

Dispersion  distinct. 

Pleochroism :  Z  pale  green,  Y  pale  brown,  X  clear 
brown,  for  common  hornblende. 
Inclusions  abundant  but  not  characteristic.     Rutile 
common. 

Alteration :  By  ordinary  weathering  to  chlorite  often 
accompanied  by  epidote,  calcite  and  quartz.  Sometimes 
to  biotite.  By  heat  to  augite. 

Differentiation :  From  other  amphiboles,  by  stronger 
color  and  pleochroism,  higher  interference  colors. 

From  pyroxenes,  see  under  Amphibole  group. 
Occurrence:     Widespread  in  igneous,  regional  meta- 
morphic  and  contact  rocks.    Hornblende  schists. 
RIEBECKITE. 

Composition:     nNaFeSi2O6.    FeSi03. 
Criteria  for  determination  in  thin  section : 
Form :    Similar  to  hornblende. 


DESCRIPTION  OF  ROCK-MAKING  MINERALS  105 

Optical  Properties :    Biaxial  and  negative. 

Color :     Dark  blue  to  black. 

Refringence  same  as  hornblende. 

Birefringence  weak  (0.005).     Interference  colors 
masked  by  absorption. 

Optic  angle  parallel  to  010.    Angle  large. 

Pleochroism  intense.     Z  is  yellowish  green,  Y  is 
blue,  X  is  indigo  blue,  nearly  black. 

Absorption  marked. 

Dispersion  strong. 

Differentiation:     Characterized  by  intense  color  and 
pleochroism,  strong  dispersion  and  pronounced  color. 

Occurrence :    In  soda-rich  igneous  rocks  and  in  some 
metamorphic  rocks. 

MICA  GROUP. 
Muscovite. 
Sericite. 
Biotite. 
Lepidolite. 
Phlogopite. 

Criteria  for  determination  in  thin  section : 
Form:  Monoclinic  or  pseudohexagonal.  As  scales, 
which  may  be  notched  or  jagged,  with  lateral  sections 
lath-shaped.  As  shreds,  characterized  by  perfect  basal 
cleavage  giving  thin  laminae.  Plates  of  hexagonal  out- 
line with  the  planes  001,  110,  010,  with  angles  of  60 
and  90  degrees.  Twinning  common  after  the  mica  law 
in  a  plane  perpendicular  to  001  and  practically  parallel 
to  110.  Zonal  structure  common  in  the  dark  varieties. 
Elongation  parallel  to  the  cleavage  and  the  c  axis. 
Optical  Properties :  Biaxial  and  negative. 

Colors  given  under  each  variety. 
Refringence  medium.    Relief  distinct. 

Birefringence  very  strong,  particularly  in  the  col- 


106  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

ored  micas,  varying  from  0.037  to  0.05.    Interference 
colors  of  the  third  order,  which  may  be  very  brilliant 
in  thin  sections  of  colorless  mica,  often  appearing 
iridescent.  Occasionally  masked  by  strong  absorption. 
Extinction  about  parallel  to  the  cleavage  lines. 
Very  small  extinction  angles  may  be  noticed  in  biotite. 
Absorption  strong  in  colored  micas. 
Optic  angle  large  in  white  micas  and  small  in  the 
ferro-magnesian  varieties,  appearing  almost  uniaxial. 
Differentiation:     Characterized    by    distinct    relief, 
strong  birefringence    (chlorite  has  weak),  one  perfect 
cleavage  marked  by  parallel,  fine  lines,  practically  par- 
allel   extinction,    mottled  appearance    between    crossed 
nicols,  maximum  extinction  in  colored  varieties  parallel 
to  the  vibration  plane  of  the  polarizer. 

MUSCOVITE. 

Composition:   H2(K,  Na)  AL(Si04),. 

Criteria  for  determination  in  thin  section : 

See  also  under  Mica  group. 

Colorless  in  thin  section. 

Inclusions:  Not  as  common  as  in  biotite.  Zircon, 
apatite,  spinel,  garnet,  quartz,  and  magnetite. 

Alteration :  By  ordinary  weathering  to  sericite,  ser- 
pentine, talc. 

Occurrence:  Most  common  of  the  micas.  Normal 
constituent  of  igneous  rocks,  especially  granites.  Abun- 
dant in  gneisses  and  schists.  Present  in  veins.  Occurs 
as  an  alteration  product  of  the  feldspars,  nephelite,  etc. 

Differentiation:     From  talc,  by  large  optic  angle. 
From  kaolinite,  by  strong  birefringence. 
From   other  micas,   by   being   colorless   in   thin 

section. 

From  chlorite,  by  strong  birefringence  and  lack 

of  color. 


DESCRIPTION  OF  ROCK-MAKING  MINERALS  107 

SERICITE. 

Sericite  is  a  fine,  scaly  or  fibrous  variety  of  muscovite, 
with  a  greater  degree  of  hydration.  It  is  nearly  uniaxial 
in  character,  with  a  small  optic  angle. 

BIOTITE. 
Pseudohexagonal. 

Composition:    (K,H)2   (Mg,  Fe)2   (Al,  Fe)2   (SiO4)3. 
Criteria  for  determination  in  thin  section : 
See  also  under  Mica  group. 
Optical  Properties: 

Color :     Black,  green,  brown,  red,  yellow. 
Angle  of  optic  axes  almost  0  degrees  in  most  bio- 
tite  of  igneous  rocks. 

Birefringence   increases   with   increase   in   iron- 
content. 

Absorption  marked. 

Pleochroism  distinct.    Z  is  dark  to  opaque  brown, 
Y  is  the  same,  X  is  pure  yellow. 
Inclusions :     Apatite  and  zircon  common.    Pleochroic 
halos  abundant  about  inclusions. 

Alteration:  Reaily  to  chlorite,  often  accompanied 
by  the  formation  of  calcite,  epidote  and  quartz. 

Differentiation:  From  alkaline  micas  by  small  optic 
angle  color  and  distinct  pleochroism. 

From  chlorite,  by  strong  birefringence  and  color. 
From  hornblende,  by  extinction  parallel  to  cleav- 
age and  almost  uniaxial  interference  figures  in  con- 
vergent light. 

Occurrence :  Important  constituent  of  many  igneous 
rocks,  gneisses  and  schists.  Developed  by  regional  and 
contact  metamorphism. 

LEPIDOLITE. 

Composition:    (Li,  K)  A1(F,  OH),  Al(Si03)3. 
Criteria  for  determination  in  thin  section : 


108  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

See  also  under  Mica  group. 

Colorless  in  thin  section. 

Pleochroism  distinct.    Z  and  Y  pink ;  X,  colorless. 

Differentiation :  Microscopically  indistinguishable 
from  muscovite.  Pink  color  is  believed  to  be  due  to 
traces  of  manganese. 

Occurrence :  In  veins  and  dikes  in  granite,  associated 
with  cassiterite,  tourmaline,  etc. 

Uses :     As  a  source  of  lithium  salts. 

PHLOGOPITE. 

Composition:    (K,  H)3(Mg,  F),  Mg;,Al(SiO4)3. 

Criteria  for  determination  in  thin  section : 

See  also  under  Mica  group. 

Color:    Brown,  brownish  red,  green,  yellow. 

Pleochroism,  Z  and  Y  are  brownish  yellow,  X  is  col- 
orless. 

Inclusions:  Hematite,  rutile  and  tourmaline  are 
common. 

Differentiation:  From  minerals  of  other  groups, 
same  as  biotite. 

From  muscovite,  paragonite  and  lepidolite  by  color. 
From  biotite,  by  mode  of  occurrence. 

Occurrence:  Only  in  crystalline  limestones,  dolo- 
mites, and  serpentines,  associated  with  spinel,  graphite, 
etc.  Absent  in  igneous  rocks. 

Uses:     As  an  insulator  in  electrical  apparatus. 

CHLORITE  GROUP. 
Penninite. 
Clinochlore. 

Character :  Similar  to  the  micas,  with  perfect  cleav- 
age parallel  to  001,  the  basal  pinacoid.  This  cleavage 
may  not  be  noticed  in  fibrous  or  secondary  chlorite. 
Aggregates  of  small,  flat  scales  of  irregular  outline,  usu- 


DESCRIPTION  OF  ROCK-MAKING  MINERALS  109 

ally  with  a  laminated  structure.  Often  in  minute  grains 
as  a  pigment  in  other  minerals.  Twinning  common  after 
the  base  and  after  the  mica  law. 

Criteria  for  determination  in  thin  section : 

Color:  Characteristically  green,  due  to  iron  protox- 
ide, varying  from  greenish  white  to  dark  green. 

Refringence  low.  No  relief.  In  penninite,  n  is  1.579 
and  1.576.  In  clinochlore,  n  is  1.596  and  1.585. 

Birefrigence  usually  weak,  with  interference  colors 
of  the  low  first  order  gray  and  bluish  gray.  Penninite 
(0.002).  Clinochlore  (0.011). 

Extinction :  Plates  parallel  to  cleavage  show  at 
times  isotropic  characteristics.  In  other  sections,  extinc- 
tion is  apparently  parallel  to  the  cleavage.  Clinochlore 
occasionally  shows  perceptible  extinction  angles. 

Pleochroism  present  in  all  chlorites  in  green  and  yel- 
low tints,  the  green  being  parallel  to  the  cleavage. 

Maximum  absorption  always  in  the  direction  of  the 
cleavage. 

Differentiation:  From  serpentine,  by  greater  ple- 
ochroism.  From  mica,  by  weaker  birefringence.  Charac- 
teristics are  pale  green  color,  distinct  pleochroism,  low 
relief  and  weak  birefringence. 

Occurrence:  Widely  distributed,  forming  essential 
constituent  of  chlorite  schist.  Occurs  secondary  in  igne- 
ous and  metamorphic  rocks,  from  the  micas,  amphiboles, 
pyroxenes,  and  garnets. 

PENNINITE. 
Pseudorhombohedral. 
Composition :     Hs  (Mg,  Fe) ,  Al2Si,Ols. 
Differentiation   from   clinochlore:      Nearly   uniaxial 
character,  negative  sign,  very  weak  birefringence,  par- 
allel extinction. 


110  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

CLINOCHLORE. 

Composition :     Same  as  penninite. 

Differentiation  from  penninite:  Distinctly  biaxial, 
positive  sign,  higher  birefringence,  occasionally  oblique 
extinction,  common  polysynthetic  twinning. 

EPIDOTE  GROUP. 

Composition:     Ca2(Al,  Fe)2(Al,  OH)   (Si04)3. 
Criteria  for  determination  in  thin  section : 
Form:     Columnar  crystals,  nearly  always  elongated, 
parallel  to  the  b  axis.    Fibrous,  massive,  or  in  irregular 
grains  as  aggregates.    Twinning  common  parallel  to  100. 
Cleavage  parallel  to  the  basal  pinacoid,  imperfect  parallel 
to  the  orthopinacoid.     Basal  cleavage  cracks  not  very 
numerous,  and  appear  parallel  to  the  general  direction 
of  elongation. 

Optical  Properties:     Biaxial  and  negative. 
Colorless  to  orange  yellow  in  thin  section. 
Refringence  high  with  rough  surface,     n  =  1.767 
and  1.730. 

Birefringence  variable,  often  strong,  with  high 
interference  colors.  Variable  in  a  single  crystal 
(0.037). 

Extinction  parallel  to  cleavage  in  elongated  sec- 
tions. In  other  sections,  extinction  angle  varies  from 
0  to  28  degrees. 

Interference  figure  of  cleavage  flakes  show  an  axial 
bar  with  concentric  rings.  Axial  plane  at  right 
angles  to  the  elongation  of  the  crystal.  Axial  angles 
are  large. 

Pleochroism:  Z  is  colorless,  yellowish  green, 
pink;  Y  is  pale  blue  to  greenish  yellow;  X  is  colorless, 
lemon  yellow,  pale  green. 

Alteration :     Epidote  is  very  resistant  to  weathering. 
Differentiation:     From     light     colored     monoclinic 


DESCRIPTION  OF  ROCK-MAKING  MINERALS  111 

pyroxenes,  by  having  optic  plane  at  right  angles  to  the 
principal  cleavage  cracks,  which  are  parallel  to  the  direc- 
tion of  elongation.  Epidote  is  characterized  by  form  and 
color,  high  refringence,  parallel  extinction  in  longitudinal 
sections,  strong  birefringence  variable  in  a  single  crystal. 
Occurrence:  Very  common,  especially  in  schists  and 
in  zones  produced  by  contact  metamorphism  between 
granites  and  limestones.  Also  as  an  alteration  product 
of  the  ferro-magnesian  minerals  and  feldspars  in  igneous 
rocks. 

ZOISITE. 

(Orthorhombic  member  of  the  Epidote  group.) 
Composition :     Same  as  epidote  without  the  iron. 
Criteria  for  determination  in  thin  section: 
Form:     Prismatic  crystals  or  granular  aggregates. 
Lamellar,  fibrous  or  in  compact  masses.     Perfect  cleav- 
age, parallel  to  010;  difficult,  parallel  to  100.     Longer 
individuals  show  transverse  parting.     Microscopic  twin- 
ning in  polysynthetic  bands  occur. 

Optical  Properties :     Biaxial  and  positive. 

Colorless  to  yellow  tints.    Usually  lacks  color. 
Refringence  high,  with  rough  surface,    n  =  1.702 
and  1.697. 

Birefringence  weak,  with  grayish  or  whitish  inter- 
ference colors  (0.005) . 

Extinction  always  parallel. 

Differentiation:  From  epidote,  by  its  lack  of  color 
and  its  weaker  birefringence.  It  is  characterized  by 
parallel  extinction,  high  relief,  very  weak  birefringence, 
strong  dispersion. 

Occurrence:  In  crystalline  schists,  associated  with 
amphibole,  particularly  hornblende.  In  igneous  rocks, 
as  an  alteration  of  the  feldspars.  In  veins  in  altered 
basic  igneous  rocks  with  quartz. 


112  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

KAOLINITE. 

Composition :     H4ALSi209. 
Criteria  for  determination  in  thin  section: 
Form:     Pseudohexagonal,   in  thin  plates  or  scales. 
Usually  in  clay-like  masses. 

Optical  Properties.     Negative. 

Colorless  in  thin  section.    Aggregates  are  cloudy. 
Refringence  low  with  no  relief,    n  =  1.563. 
Birefringence  weak  (0.007). 

Differentiation:  From  muscovite  and  talc  by  weak 
birefringence. 

Occurrence:  Kaolinite  is  the  most  common  second- 
ary mineral.  It  is  derived  from  the  feldspars  by  ordi- 
nary weathering.  Occurs  in  large  sedimentary  clay 
masses  as  a  result  of  the  decomposition  of  aluminous 
silicates. 

Uses:  It  is  used  in  the  manufacture  of  porcelain, 
pottery,  and  china. 

TITANITE  (SPHENE). 

Composition :     CaTiSi05. 

Criteria  for  determination  in  thin  section : 

Form:  In  detached  crystals  and  in  disseminated 
grains.  Often  wedge-shaped  when  primary,  and  irreg- 
ular grains  when  secondary.  Flattened  parallel  to  the 
base.  Elongated  parallel  to  a  or  c.  Cleavage  imperfect, 
parallel  to  the  prism,  appearing  as  a  few  rough  cracks. 
Cleavage  rarely  observed  in  secondary  forms.  Twinning 
seen  only  between  crossed  nicols,  the  twinning  boundaries 
bisecting  the  acute  angles  of  the  rhombs. 

Optical  Properties : 

Colorless,  brownish  or  yellowish. 

Refringence  high,  with  rough  surface,    n  =  2.009 

and  1.888. 


DESCRIPTION  OF  ROCK-MAKING  MINERALS  113 

Birefringence  extremely  strong,  with  interference 
colors  of  a  high  order,  like  those  of  calcite  (0.1214). 
Pleochroism  strong  in   deeply  colored  varieties, 
appearing  yellowish,  parallel  to  a  and  reddish  par- 
allel to  c. 

Optic  plane  parallel  to  010,  with  a  small  optic 
angle. 
Inclusions:     Often  grouped  about  the  center  of  the 

crystal. 

Alteration:    To  a  light  yellow  amorphous  mass  with 

calcite. 

Differentiation:     From  staurolite,  by  the  fact  that 

the  optic  plane  is  in  the  shorter  diagonal  of  the  cross 

section  instead  of  in  the  longer.     It  is  characterized  by 

high  relief,  extreme  birefringence,  biaxial  character  and 

positive  sign. 

Occurrence:     Widely    distributed    as    an    accessory 

mineral  in  igneous  rocks.    Occurs  in  schists  and  gneisses. 

Is  found  as  a  secondary  product  called  leucoxene,  derived 

from  ilmenite  in  basic  igneous  rocks. 

FELDSPAR  GROUP. 

Monoclinic   Feldspar. 
Orthoclase. 

Triclinic  Feldspars. 

Microcline. 

Albite. 

Oligoclase. 

Labradorite. 

Anorthite. 

Composition :    Silicates  of  aluminum  with  potassium, 
sodium,  calcium,  rarely  barium. 

Form:     Crystal  forms  are  similar,  often  short  pris- 


114  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

matic,  somewhat  flattened,  parallel  to  010.  Narrow 
bands  of  albite,  intergrown  with  orthoclase  or  microcline, 
forming  "perthite"  common.  Plagioclase  feldspars  are 
triclinic,  but  angle  alpha  varies  little  from  90  degrees. 

Cleavage :  Perfect  parallel  to  the  basal  pinacoid  and 
almost  as  perfect  parallel  to  the  clinopinacoid.  Cleavage 
cracks  usually  noticed  only  in  very  thin  sections.  The 
two  cleavages  intersect  at  90  degrees  in  orthoclase  and 
at  93  or  94  degrees  in  the  plagioclase  feldspars.  The 
cleavage  is  not  as  distinct  as  the  cleavage  of  mica  or 
hornblende. 

Twinning:  Twinning  common  in  the  feldspars  fol- 
lowing the  Carlsbad,  Mannebach,  Baveno,  Albite  and 
Pericline  laws. 

CARLSBAD  TWINNING  is  the  simplest  type  of  feldspar 
twinning,  and  it  occurs  in  both  monoclinic  and  triclinic 
varieties,  in  the  latter  case  causing  confusion  with  other 
types  of  twinning.  The  twinning  plane  is  the  orthopina- 
coid  and  the  composition  face  is  the  clinopinacoid.  Carls- 
bad twins  always  consist  of  two  individuals,  a  fact  which 
may  be  used  to  differentiate  between  the  plagioclase  feld- 
spars and  orthoclase. 

MANNEBACH  TWINNING:  The  basal  pinacoid  is  the 
composition  face  and  twinning  plane.  This  type  of  twin- 
ning is  not  common. 

BAVENO  TWINNING  :  The  twinning  plane  is  the  clino- 
dome  to  which  the  twinning  axis  is  normal.  Sections 
cutting  such  a  twin  show  square  or  rhombohedral  out- 
lines, the  cleavages  being  parallel  to  the  sides. 

ALBITE  AND  PERICLINE  TWINNING  are  especially  com- 
mon on  the  plagioclase  feldspars  and  are  used  as  a  means 
of  identification.  They  are  usually  visible  to  the  naked 
eye.  In  thin  sections  they  appear  as  polysynthetic  stria- 
tions  in  narrow  alternating  light  and  dark  bands,  which 


DESCRIPTION  OF  ROCK-MAKING  MINERALS 


115 


extinguish  alternately  upon  being  rotated.  In  the  albite 
type  of  twin,  the  twinning  axis  is  normal  to  the  clinopina- 
coid.  Hence  the  lamellae  are  parallel  to  the  clinopinacoid 
and  the  striations  are  visible  only  on  the  basal  pinacoid 
and  the  orthopinacoid. 

In  the  pericline  twinning,  the  twinning  axis  is  parallel 
to  6.  Therefore,  the  pericline  striations  are  visible  on  all 
faces  of  the  crystal.  It  is  obvious  that  if  twinning  occurs 
on  the  clinopinacoid,  it  must  be  of  the  pericline  type.  In 
thin  section,  the  pericline  twinning  is  visible  in  any  sec- 
tion except  a  section  cut  parallel  to  the  composition  face. 


Fig.  19.     Triclinic  feldspar  form,  showing  the  positions  of  the 
characteristic  albite  and  pericline  striations. 

Differentiation    of     the     Feldspars    by     Twinning. — 

Orthoclase  occurs  in  simple  twins,  after  the  Carlsbad, 
Baveno  and  Mannebach  laws,  but  never  in  polysynthetic 
twins. 

Microcline  is  always  polysynthetically  twinned  in  two 
directions,  a  combination  of  albite  and  pericline  twinning 
producing  a  rectangular  crosshatching  between  crossed 
nicols. 


116  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

Plagioclase  feldspars  practically  always  show  a  poly- 
synthetic  twinning,  after  the  albite  law. 

Albite  shows  twinning  lines  that  are  fine  and  far 
apart,  irregular  and  interrupted. 

Oligoclase  shows  twinning  lines  that  are  clear  and 
of  regular  widths. 

Labradorite  shows  twinning  lamellae  which  are  clear 
and  definite,  but  the  width  often  varies  from  one  lamella 
to  another. 

Anorthite  shows  twinning  lamellae  which  are  broad 
and  regular,  after  the  albite  law,  while  those  of  the  peri- 
cline  law  are  distributed  only  in  certain  of  the  albite 
bands. 

Optical  Properties. — The  optic  plane  containing  the 
optic  axes  and  the  bisectrices  is  the  chief  optic  element. 
Its  position  in  each  of  the  feldspars  has  definite  relations 
to  the  cleavage,  external  faces,  axes,  and  the  positions 
of  the  albite  twinning.  In  orthoclase  and  microcline, 
for  example,  the  optic  plane  is  almost  parallel  to  the 
basal  pinacoid,  hence  agrees  with  the  direction  of  the 
most  perfect  cleavage.  Z  is  perpendicular  to  the  clino- 
pinacoid.  X  lies  in  the  plane  of  the  clinopinacoid  almost 
parallel  to  the  base,  varying  about  5  degrees  in 
microcline. 

Refringence  is  low,  similar  to  that  of  quartz.  The 
Becke  test  is  advised. 

Birefringence  is  weak,  similar  to  that  of  quartz. 

TABLE  OF  REFRINGENCE  OF  THE  FELDSPARS. 

n  n  n 

g  m  p 

Orthoclase         .         .         .  1.526  1.5237  1.518 

Microcline     .          .  .    -  .          .  1.5296  1.5264  1.5224 

Albite       ....  1.54  1.534  1.531 

Oligoclase     ....  1.5469  1.5431  1.5389 

Labradorite       .          .          .  1.5625  1.5578  1.5548 

Anorthite      ....  1.5884  1.5837  1.5757 


DESCRIPTION  OF  ROCK-MAKING  MINERALS  117 

TABLE  OF  BIREFRINGENCE  OF  THE  FELDSPARS. 

Section       Normal  Parallel  to 
Normal  to  X      to  Z     Optic  Plane 

n  —n         n  —n  n  —n 

g         in  m      p  g       p 

Orthoclase         .          .          .  0.0023  0.0047  0.007 

Microcline     ....  0.0032  0.004  0.0072 

Albite       .         .         .         .  0.006  0.003  0.009 

Oligoclase     ....  0.0038  0.0042  0.008 

Labradorite       .         .         .  0.0047  0.003  0.0077 

Anorthite      ....  0.0047  0.008  0.0127 

Alteration:  In  zone  of  weathering  to  kaolinite, 
quartz,  and  calcite.  The  alteration  of  the  feldspars  to 
kaolinite  or  to  other  closely  associated  hydrous  aluminum 
silicates  is  the  ordinary  method  of  origin  of  clay,  and 
takes  place  more  frequently  in  the  acid  than  in  the  basic 
feldspars  by  a  leaching  out  of  the  potash  and  hydration. 
The  alteration  begins  along  cleavage  cracks,  and  finally 
spreads  over  the  entire  feldspar,  causing  it  to  appear 
opaque  or  cloudy.  The  kaolinite  is  usually  in  small  flakes. 

The  alteration  of  the  acid  feldspars  to  sericite,  a 
variety  of  muscovite,  is  common,  the  alteration  taking 
place  first  along  cleavage  cracks.  This  is  usually 
accomplished  through  the  agency  of  hot  solutions. 

The  basic  feldspars  frequently  alter  to  chlorite,  also 
to  epidote  associated  with  quartz  and  calcite. 

Occurrence:  Feldspars  are  the  most  abundant  and 
the  most  widely  distributed  minerals  of  the  earth's  crust, 
occur  abundantly  in  metamorphic  rocks,  and  frequently 
in  sedimentary  rocks.  Also  in  veins. 

Differentiation :  From  quartz,  by  presence  of  cleavage 
and  twinning,  and  biaxial  character.  Characterized  by 
frequency  of  occurrence  in  practically  all  conditions,  low 
refringence,  and  weak  birefringence.  Also  by  readiness 
with  which  they  alter.  To  distinguish  one  feldspar  from 
another  the  twinning,  extinction  angles,  optic  sign  and 
refringence  as  determined  by  the  Becke  test  are  aids. 


118  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

To  distiguish  one  plagioclase  feldspar  from  another, 
several  practical  methods  have  been  devised. 

1.  EXTINCTION    ANGLES    ON    BASE    AND    BRACHY- 

PINACOD). 

Schuster  established  relations  existing  between  the 
extinction  angles  on  the  base  and  the  brachypinacoid. 
The  prevalence  of  favorable  cleavages  aids  in  this  deter- 
mination. As  these  minerals  are  all  triclinic,  extinction 
takes  place  in  all  sections  unsymmetrically  with  respect 
to  crystallographic,  twinning  or  cleavage  lines.  Conse- 
quently, extinction  angles  will  always  be  observed.  When 
the  extinction  angles  on  both  the  basal  pinacoid  and 
the  brachypinacoid  are  large,  anorthite  is  in  all  prob- 
ability the  mineral  observed.  When  the  angles  are  both 
small,  the  feldspar  is  oligoclase.  Albite  and  labradorite 
show  intermediate  extinction  angles.  Orthoclase  has 
extinction  on  the  basal  pinacoid  of  from  5  to  9  degrees. 

The  extinction  angles  given  in  the  following  table 
are  marked  plus  or  minus.  The  angles  on  the  base  and 
brachypinacoid  are  marked  plus  when  the  direction  of 
extinction  has  apparently  moved,  as  the  hands  of  a  watch, 
with  reference  to  the  upper  right-hand  edge  of  the  crys- 
tal, between  the  base  and  pinacoid.  The  angles  are 
marked  minus  when  the  reverse  is  true. 


ANGLES. 

Section  parallel  to  base         Section  parallel  to  brach- 

measured    from    trace    of  ypinacoid   measured    from 

pinacoidal  cleavage.  trace  of  basal  cleavage. 

Albite    ............     4  Albite  ............     20 

Oligoclase   .........     2  Oligoclase   ........       7 

Labradorite     ......  —  51/2  Labradorite    ......  —  20 

Anorthite   ........  —  37  Anorthite   .  ..—  42 


DESCRIPTION  OF  ROCK-MAKING  MINERALS         119 

2.  STATISTICAL  METHOD. 

The  method  proposed  by  Michel  Levy  is  practical 
in  all  sections,  showing  the  albite  twinning.  This 
method  consists  in  finding  the  maximum  equal  extinc- 
tions on  opposite  sides  of  an  albite  twinning  line.  The 
position  of  the  plane  which  gives  maximum  extinctions 
in  the  zone  normal  to  the  brachypinacoid  is  different  for 
different  feldspars.  This  method,  though  tedious,  is 
reliable,  in  that  the  various  species  have  characteristic 
maxima. 

Sections  normal  to  the  brachypinacoid  may  be 
recognized  by  the  fact  that  the  twinned  parts  show  equal 
illumination  eight  times  upon  a  complete  rotation  of  the 
stage,  once  every  45  degrees,  in  which  position  the 
two  parts  seem  to  belong  to  one  individual.  The  faintly 
discernible  twin  line  must  be  parallel  to  the  plane  of 
vibration  of  either  of  the  nicols  at  equal  illumination. 

Maximum  extinction  angles  in  sections  perpendicular 
to  albite  twinning: 

Albite 16 

Oligoclase    2 

Labradorite    34 

Anorthite Over  37 

Monoclinic  Feldspar. 

ORTHOCLASE. 
Composition :    KAlSisO8. 
Criteria  for  determination  in  thin  section: 
See  also  under  Feldspar  group. 

Twinning  after  Carlsbad  law  common,  after  Baveno 
and  Mannebach  less  common. 

Optical  Properties :     Biaxial  and  negative. 
Colorless  in  thin  section. 

Refringence    low.      Relief    absent    and    surface 
smooth. 


120  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

Birefringence  very  weak,  with  interference  colors 

of  the  lower  first  order,  bluish  gray,  white,  etc.,  not 

quite  as  bright  as  the  colors  of  quartz  and  plagioclase. 

Alteration  to  kaolinite  so  prevalent  that  surface 
usually  appears  cloudy. 

Differentiation:  From  other  feldspars,  see  under 
Feldspar  group. 

From  quartz,  by  cloudy  appearance,  and  negative 

character. 

Occurrence:  Abundant  in  acid  plutonic  rocks,  pres- 
ent in  intermediate  and  certain  basic  igneous  rocks,  in 
schists,  gneisses,  and  in  contact  zones.  As  perthite,  with 
bands  of  albite. 

Uses:  In  the  manufacture  of  porcelain  and  china. 
A  variety  of  orthoclase  called  moonstone  is  used  as 
a  gem. 

SANADINE. 

Sanadine  is  a  clear,  glassy  variety  of  orthoclase, 
occurring  in  rhyolite,  trachyte,  obsidian,  etc.  It  decom- 
poses less  readily  than  orthoclase,  has  a  smaller  axial 
angle,  and  usually  contains  more  inclusions. 

Triclinic  Feldspars. 

MICROCLINE. 
Composition:  KAlSi308. 
General  characters  same  as  orthoclase. 
Differentiation:     From    orthoclase;    simple   crystals 
not  showing  the  crossed  twinning  have  extinction  angles 
of  about  15  degrees  on  the  base  with  reference  to  the 
brachypinacoidal  cleavage. 

From  other  feldspars  by  the  characteristic  crossed, 
rectangular,  grating  structure. 

Occurrence:  Similar  to  orthoclase,  but  more  abun- 
dant in  pegmatites. 


DESCRIPTION  OF  ROCK-MAKING  MINERALS  121 

ALBITE. 

Composition :   NaAlSi308. 

Form  and  cleavage  characteristic  of  Feldspar  group. 

Optical  Properties :     Biaxial  and  positive. 
See  under  Feldspar  group. 

Differentiation :  From  orthoclase,  by  the  presence  of 
polysynthetic  twinning. 

From    microcline,    by    the    absence    of    grating 

structure. 

From  other  plagioclase  feldspars,  see  under  Feld- 
spar group. 

Occurrence:  Seldom  as  a  primary  constituent  of 
igneous  rock  except  as  an  intergrowth  with  orthoclase 
or  microcline  in  the  form  of  perthite  in  soda-rich  igneous 
rocks. 

OLIGOCLASE. 

Composition :    Ab6Anx  to  A^  An2. 

Differentiation :     See  under  Feldspar  group. 

Alteration:  Kaolinization  is  less  frequent  than  in 
albite. 

Occurrence:  In  eruptive  rocks  and  in  crystalline 
schists.  More  common  in  granites  than  albite  is. 

LABRADORITE. 

Composition:     AfyA^  to  AbjAn.,. 

Differentiation:     See  under  Feldspar  group. 

Alteration :  To  a  micaceous  mass.  To  an  aggregate 
composed  of  zoisite,  epidote,  albite,  quartz,  etc. 

Inclusions:  Hematite  and  ilmenite  abundant  in  col- 
ored varieties. 

Occurrence:  Common  in  basic  igneous  rocks  as 
gabbro,  basalt,  etc.,  with  olivine,  augite  and  mag- 
netite. Is  the  principal  constituent  of  anorthosite. 
Occurs  sparingly  in  meteorites. 

Uses :     As  an  ornamental  stone. 


122  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

ANORTHITE. 

Composition :     CaAl2Si2O8. 

Differentiation:  Anorthite  has  the  strongest  bire- 
fringence and  the  highest  refringence  of  all  of  the  feld- 
spars. See  under  Feldspar  group. 

Occurrence :  As  an  essential  constituent  of  basic  igne- 
ous rocks. 

Developed  by  contact  and  regional  metamorphism. 


DESCRIPTION  OF  ROCK-MAKING  MINERALS  123 


PART  TWO.— PETROGRAPHY. 

CHAPTER  8. 
General  Discussion  of  Igneous  Rocks. 

Petrography  is  that  division  of  Petrology  which  is 
concerned  with  the  systematic  classification  and  descrip- 
tion of  rocks  megascopically  and  microscopically. 

The  broad  classification  of  rocks  according  to  origin 
is:  1,  Igneous;  2,  Sedimentary,  and  3,  Metamorphic. 

Igneous  rocks  are  those  which  have  solidified  by  cool- 
ing from  a  molten  condition. 

Sedimentary  rocks  are  those  which  have  been  depos- 
ited under  water  or  on  land  by  mechanical,  chemical  or 
organic  processes. 

Metamorphic  rocks  are  those  which  are  derived  from 
previously  existing  igneous  or  sedimentary  rocks  by  heat 
alone  or  by  pressure  and  resultant  heat. 

Igneous  Rocks. 

Classification. — Several  methods  for  classifying  igne- 
ous rocks  have  been  devised,  two  of  which  seem  to  be 
more  or  less  satisfactory  for  practical  purposes.  These 
methods  are  the  qualitative  and  the  quantitative  classi- 
fications. By  an  examination  of  the  minerals  comprising 
the  rock  many  inferences  may  be  derived  as  to  the  mode 
of  origin,  the  conditions  of  crystallization,  the  general 
chemical  composition,  whether  acid  or  basic,  etc.  A 
macroscopic  observation  alone  gives  the  observer  some 
basis  for  a  rough  classification.  In  the  field,  the  mining 


124  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

engineer  or  geologist  may  find  a  rock  which  is  essentially 
quartz-  and  feldspar,  with  a  small  percentage  of  ferro- 
magnesian  minerals.  He  may  call  it  a  granite.  His  clas- 
sification is  correct  if  the  rock  contains  25  or  30  per 
cent  of  quartz.  But  if  it  contains  only  a  few  per  cent, 
he  will  hesitate  as  to  whether  the  rock  is  a  granite  or  a 
syenite. 

As  long  as  such  a  doubt  exists  as  to  the  proper  clas- 
sification of  a  rock,  it  is  obvious  that  the  system  of 
classification  is  at  fault.  It  is  of  course  clear  that  all 
possible  gradations  in  mineral  percentages  exist  between 
the  various  igneous  rock  types.  In  so  far  as  this  is 
true,  a  qualitative  classification  based  upon  mineral  per- 
centages is  defective.  On  the  other  hand,  this  method 
is  exceedingly  rapid  in  that  one  who  is  skilled  in  the 
manipulation  of  the  microscope  and  in  the  interpretation 
of  the  phenomena  observed  in  thin  section  can  infer  much 
from  a  glance  about  the  nature  of  the  rock. 

For  more  complete  descriptions  of  a  rock,  the  quanti- 
tative classification  is  more  satisfactory  in  that  the  chem- 
ical composition  of  the  rock  is  used  as  a  basis  for  clas- 
sification. But  such  an  analysis  usually  takes  two  or  three 
days  of  careful  work  by  a  skilled  chemist.  Obviously,  a 
qualitative  classification  with  the  aid  of  the  microscope 
meets  the  requirements  of  the  great  majority  of  cases 
generally  met  with. 

Essential  and  Accessory  Minerals. — Of  the  thousand 
minerals  which  are  known,  only  about  ninety  occur  in 
igneous  rocks.  Twenty-five  of  these  are  of  prime  impor- 
tance in  determining  the  classification  of  a  rock.  These 
are  the  "essential  minerals,"  for  their  presence  is  essen- 
tial to  the  classification  and  definition  of  the  rock  type  in 
which  they  appear.  The  remaining  minerals,  which  com- 
prise the  majority,  are  the  "accessory  minerals,"  whose 


GENERAL  DISCUSSION  OF  IGNEOUS  ROCKS  125 

presence  or  absence  does  not  influence  the  name  under 
which  the  rock  is  classified.  They  are  usually,  though 
not  always,  present  in  small  quantities.  Typical  acces- 
sory minerals  are  zircon,  apatite,  ilmenite,  titanite,  etc. 

It  is  to  be  noted  that  minerals  which  are  not  essen- 
tial to  the  definition  of  a  large  division,  as  the  granite 
group,  may  become  essential  if  the  group  is  subdivided 
into  a  smaller  division,  as  the  amphibole-granite  class. 

Primary  and  Secondary  Minerals. — Primary  minerals 
are  those  which  crystallized  out  from  solution  at  the 
time  of  the  solidification  of  the  magma.  Examples  are  the 
feldspars  and  quartz  in  granite.  Secondary  minerals  are 
those  which  have  formed  after  the  solidification  of  the 
magma  by  the  alteration  of  the  previously  existing  min- 
erals, the  alteration  usually  taking  place  through  the 
agency  of  weathering.  Examples  are  the  alteration  of 
the  feldspars  to  kaolinite  and  muscovite,  and  of  the 
amphiboles  and  the  pyroxenes  to  serpentine.  Secondary 
minerals  are  derived  from  primary  minerals  by  the  appli- 
cation of  heat  and  pressure.  Sericites  are  thus  derived 
from  impure  quartzites;  chlorite  from  amphiboles  and 
pyroxenes;  talc  from  amphiboles,  pyroxenes  and  impure 
dolomites. 

Texture. — Although  composition  is  the  chief  means 
of  distinguishing  rock  types  according  to  the  qualitative 
system,  texture  likewise  plays  an  important  role  in  that 
it  furnishes  an  important  clue  as  to  the  circumstances 
under  which  the  rock  was  formed.  There  are  thus  two 
considerations  to  be  taken  into  account  in  the  identifi- 
cation of  a  rock  —  the  chemical  composition,  and  the 
texture. 

Texture  is  defined  as  the  size,  shape  and  mode  of 
aggregation  of  the  constituent  particles  of  a  rock.  It 
is  determined  by  certain  conditions  prevailing  in  and 


126  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

about  the  molten  magma  at  the  time  of  the  solidification 
of  the  rock  mass.    Most  important  of  these  are : 

1.  The  rate  of  cooling; 

2.  The  chemical  composition  of  the  magma; 

3.  Pressure; 

4.  Temperature; 

5.  Action  of  mineralizers  as  steam,  HC1,  Fl,  B. 

The  rock  solidifying  at  great  depths  cools  very  slowly, 
allowing  the  minerals  time  to  crystallize  into  well-formed 
individuals.  Many  minerals  crystallize  simultaneously, 
and  these  minerals  interfere  with  each  other  as  they 
grow.  The  interpenetration  or  irregular  boundary  line 
between  any  two  crystals  is  a  mutual  adjustment  of 
simultaneous  formation.  Molten  magmas  which  are 
suddenly  subjected  to  rapid  cooling,  such  as  would  accom- 
pany an  extrusion  on  or  near  the  surface,  crystallize  rel- 
atively rapidly,  with  the  result  that  a  portion  of  the  rock 
mass  crystallizes  as  a  glass.  Microscopic  crystals  usu- 
ally have  time  to  make  their  appearance.  It  is  also  found 
that  crystals  of  some  minerals  grow  more  rapidly  than 
crystals  of  other  minerals.  When  a  rock  shows  a  glassy 
appearance,  with  minute  crystals  embedded  in  the  glass, 
the  glass  is  regarded  as  a  "groundmass." 

The  common  textures  may  be  reduced  to  four,  as 
follows : 

1.  Glassy; 

2.  Felsitic,  or  stony ; 

3.  Porphyritic; 

4.  Granitoid. 

Glassy  texture  is  characterized  by  absence  of 
crystallization. 

Felsitic  or  stony  texture  shows  some  crystallization  of 
minute  crystals  enmeshed  in  a  glassy  or  dense  ground- 


GENERAL  DISCUSSION  OF  IGNEOUS  ROCKS  127 

mass,  giving  the  rock  a  stony  or  noncrystalline 
appearance. 

Porphyritic  texture  results  from  conditions  within 
the  magma  which  allow  the  crystallization  of  certain  min- 
erals to  take  place  before  any  other  appears.  These  well- 
defined  crystals,  which  are  called  "phenocrysts,"  are 
embedded  in  a  finer  ground  mass,  which  may  be  wholly 
glassy,  partly  crystalline,  or  very  finely  crystalline 
throughout. 

Granitoid  texture  is  applied  to  those  rocks  which  con- 
tain no  groundmass  and  which  are  composed  of  crystals 
of  the  same  general  time  of  growth  or  which  separated 
out  in  order  of  their  basicity.  In  this  case  the  earlier 
minerals  show  well-defined  boundaries  and  crystal  planes, 
whereas  the  later  minerals  fill  the  interstices  and  assume 
an  irregular  shape,  determined  by  the  position  of  the 
earlier  minerals. 

Extrusive  flows  and  intrusive  lavas  and  dikes  are 
usually  characterized  by  the  presence  of  a  groundmass. 
The  deep-seated  rocks  are  characterized  by  a  granitoid 
texture.  All  gradations  in  texture  between  rocks  con- 
sisting entirely  of  glass  and  of  wholly  crystalline  mate- 
rial exist. 

To  illustrate  the  use  of  mineral  composition  and  tex- 
ture in  classifying  rocks,  the  following  examples  are 
given : 

ROCK.  MINERALS.  TEXTURE- 

Granite —  Alkali  feldspar  and  quartz  .         Granitoid. 
Syenite —                 same         .          .          .          .    Groundmass  present. 

Diorite —  Acid  feldspars         .          .  .         Granitoid. 
Andesite —              same         .          .          .          .    Groundmass  present. 

Gabbro — -  Lime  feldspars       .          .  .         Granitoid. 
Basalt —                  same         .          .  .    Groundmass  present. 

Diabase —  same  but  with  intermediate  texture. 


128  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

Textural  Terms. — Convenient  terms  which  are  applied 
to  igneous  rocks  to  describe  the  amount  of  crystallized 
matter  present,  are : 

1.  Glassy,  in  which  no  crystals  are  present ; 

2.  Cryptocrystalline,  in  which  crystals  are  present 
but  visible  neither  to  the  eye  nor  to  the  microscope ; 

3.  Microcrystalline,  in  which  crystals  are  present  but 
visible  only  under  the  microscope ; 

4.  Hypocrystalline,  in  which  the  rock  consists  partly 
of  glass  and  partly  of  crystallized  matter ; 

5.  Holocrystalline,  in  which  the  rock  is  completely 
crystallized  and  no  glass  exists. 

A  classification  of  terms  which  describe  the  form  and 
shape  of  the  crystals : 

1.  Idiomorphic  crystals  are  those  which  have  their 
own  peculiar  geometric  form.    The  first  minerals  to  crys- 
tallize from  any  solution  are  idiomorphic,  as  they  were 
allowed  to  grow  without  interference. 

2.  Hypidiomorphic  crystals  are  those  having  part  of 
their  planes   present  and   part   absent.     This   may  be 
brought  about  by  an  overlap  in  the  time  of  crystallization 
of  a  series  of  minerals.     One  mineral  is  not  given  time 
to   crystallize    completely    before    an  adjacent    mineral 
interferes. 

3.  Allotriomorphic   crystals   are  those   in   which   no 
crystallographic  planes  are  present.    This  is  true  of  the 
last  minerals  to  crystallize.     Their  shape  is  determined 
by  previously  existing  minerals.     Simultaneous  crystal- 
lization sometimes  results  in  the  development  of  allotrio- 
morphic  crystals. 

Rosenbusch's  Law. — There  is  a  normal  order  of  crys- 
tallization in  igneous  rocks  which  in  general  is  a  law  of 


GENERAL  DISCUSSION  OF  IGNEOUS  ROCKS  129 

decreasing  basicity,  and  is  determined  by  the  presence 
or  absence  of  silica,  the  chief  acid  radicle,  in  rock-form- 
ing minerals.  It  was  first  worked  out  by  Rosenbusch 
(Heidelberg)  and  is  briefly  as  follows: 

1.  Minor  accessories:     Apatite,  magnetite,  hematite, 
ilmenite,  pyrite,  chalcopyrite,  pyrrhotite,  zircon,  titanite, 
garnet. 

2.  Ferro-magnesian  minerals:    Olivine,  orthorhombic 
pyroxenes,  monoclinic   pyroxenes,     amphiboles,    biotite, 
muscovite. 

3.  Feldspathic    minerals:    Plagioclase    feldspars    in 
order,   from   anorthite  through   bytownite,   labradorite, 
andesine,  oligoclase,  to  albite;  orthoclase,  feldspathoids, 
nephelite,  leucite,  sodalite.     These  latter  minerals  may 
crystallize  out  before  or  after  the  feldspars. 

4.  Quartz,  microcline.  Quartz  sometimes  shows  inter- 
growths  with  orthoclase. 

Volcanic  and  Plutonic  Rocks. — Igneous  rocks  are 
finally  divided  into  two  important  types,  which  depend 
directly  upon  mode  of  occurrence. 

1.  Volcanic  or  eruptive  rocks  are  those  which  flow 
out  upon  the  surface  or  are  ejected  into  the  upper  crust 
of  the  earth  near  enough  to  the  surface  to  assume  a  tex- 
ture characteristic  of  rapid  cooling.    Volcanic  rocks  have 
a  glassy  or  porphyritic  texture. 

They  may  be  either  holocrystalline  porphyritic  or 
hypocrystalline  porphyritic. 

2.  Plutonic  or  intrusive  rocks  are  those  which  do  not 
reach  the  surface  except  by  subsequent  erosion  of  the 
overlying  strata.     They  take  on  a  texture  characteristic 
of  slow  cooling.     Plutonic  rocks  are  therefore  holocrys- 
talline   and    granitoid,    occasionally    porphyritic.     They 
may  be  distinguished  from  volcanic  rocks  by  the  absence 
of  groundmass. 


130  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

Geological  Occurrence. — The  following  table  lists  the 
commonly  observed  original  structures  of  igneous  rocks. 

VOLCANIC  ROCKS. 

Extrusive. 

1.  Pyroclastic  or  f ragmen tal  deposits,  as  ash  or  tuff. 

2.  Volcanic  necks. 

3.  Lava  flows  or  sheets.    Overflow  from  fissures. 

VOLCANIC  OR  PLUTONIC  ROCKS. 
Intrusive. 

4.  Intrusive  sheets  or  sills. 

5.  Bysmaliths. 

6.  Laccoliths. 

7.  Dikes. 

PLUTONIC  ROCKS. 
Intrusive. 

8.  Bosses  or  stocks. 

9.  Batholiths. 

Petrogeny. 

Magma. — A  magma  is  a  fused  rock  mass  in  mutual 
solution.  The  essential  feature  of  a  solution  is  its  ten- 
dency to  become  homogeneous.  This  tendency  is  pro- 
duced by  diffusion,  convection  currents,  differences  in 
temperature,  sinking  of  fragments  of  superincumbent 
rocks,  etc. 

Magmas  do  not  originate  in  the  places  where  they  are 
now  observed.  They  move 

1.  In  the  zone  of  flow : 

a.  By  rising  gradually,  like  a  bubble  of  air  in  wa- 
ter, with  a  flowage  of  the  rocks  above  so  as  to  allow 
passage ; 

b.  By  overhead  stoping  and  absorption ; 

c.  By  assimilation. 


GENERAL  DISCUSSION  OF  IGNEOUS  ROCKS  131 

2.  In  the  zone  of  fracture : 

a.  By  following   the   course   of   least   resistance 
through  whatever  openings  exist. 

b.  By  overhead  stoping. 

Differentiation. — The  possible  causes  of  differentia- 
tion in  a  still  fluid  magma  are  gravity  and  differences  in 
temperature,  of  which  gravity  is  by  far  the  more  impor- 
tant. This  accounts  for  the  accumulation  and  concen- 
tration of  magnetite  along  the  lower  border  of  a  magma. 
It  is  the  first  mineral  to  crystallize.  • 

Magmatic  Stoping. — Marginal  assimilation  is  one  of 
the  methods  of  magma  advance  through  overlying  rock 
formations.  This  method  is  effective  chiefly  in  the  early 
part  of  the  magma's  history  and  takes  place  at  the  main 
contacts  and  along  a  relatively  limited  surface. 

According  to  the  theory  of  magmatic  stoping,  each 
batholithic  magma  in  its  gradual  advance  upward 
through  the  overlying  rocks  engulfs  large  blocks  of  rock 
from  the  roof  and  walls.  This  process  is  facilitated  by 
the  shattering  which  it  is  believed  accompanies  an  intru- 
sion, due  to  unequal  heating  of  the  country  rock  along 
the  contacts.  These  blocks  are  thus  dissolved  at  depths 
forming  a  compound  magma  by  assimilation.  The  aver- 
age crust  rock  is  more  soluble  in  basic  rocks  than  in  acid. 

Crystallization. — A  eutectic  is  that  proportion  of  two 
or  more  substances  that  has  the  lowest  freezing  point 
for  those  substances.  Eutectic  aggregates  represent  later 
products  of  crystallization  because  the  first  mineral  to. 
crystallize  is  that  which  is  in  excess  as  compared  with 
certain  standard  proportions.  Thus,  an  intimate  inter- 
growth  of  quartz  and  feldspar  is  a  proof  of  simultaneous 
crystallization. 

If  a  third  substance  were  added  to  a  eutectic  propor- 
tion, it  would  lower  the  temperature  so  as  to  approach 


132  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

a  ternary  eutectic,  unless  the  third  substance  were  pres- 
ent to  an  amount  less  than  one  per  cent,  in  which  case 
it  may  be  considered  negligible.  A  mineral  which  crys- 
tallizes late  has  an  appreciable  effect  on  a  eutectic  pro- 
portion. One  which  crystallizes  early,  as  apatite,  has  no 
effect.  Thus  the  importance  of  an  accessory  mineral 
depends  upon  its  solubility  in  a  eutectic,  although  it  is 
usually  present  in  such  a  small  amount  that  it  may  be 
disregarded. 

On  the  other  -hand,  the  gases  will  be  present 
throughout  the  crystallization  of  the  magma.  They  tend 
to  lower  the  temperature  of  crystallization  to  a  greater 
extent  than  do  the  accessory  minerals,  and  they  aid  the 
magma  to  solidify  to  a  crystalline  mass  instead  of  to  a 
glass. 

In  perfect  isomorphism,  A  and  B  form  mixed  crystals 
in  any  proportion,  so  that  there  is  a  complete  series  of 
possible  varieties  between  end  members.  Such  a  series 
is  obtained  between  minerals  which  agree  very  nearly 
in  molecular  volume  and  crystalline  elements.  The  albite- 
anorthite  series  is  an  example. 

In  imperfect  isomorphism  only  certain  mixtures  are 
possible,  as  A  with  some  B,  or  B  with  some  A.  The  ortho- 
clase-albite  series  is  an  example.  Orthoclase  may  con- 
tain some  albite,  but  no  continuous  series  of  mixtures 
connects  pure  orthoclase  and  pure  albite. 

Influence  of  Gases  on  a  Magma. — Gases  present  in 
magmas  are  in  a  condition  of  unstable  equilibrium,  par- 
ticularly at  slight  depths,  liberating  heat  by  reaction  with 
each  other.  With  decrease  in  pressure  the  reaction 
between  gases  increases.  Thus  it  was  observed  in 
Hawaii  that  the  temperature  of  a  lava  lake  changed  a 
few  hundred  degrees  in  temperature  as  the  amount  of 
gases  passing  through  it  increased  or  decreased.  The 


GENERAL  DISCUSSION  OF  IGNEOUS  ROCKS  133 

temperature  increased  with  increase  in  the  amount  of 
gases. 

Relation  Between  Composition  of  Igneous  Rocks  and 
Magmas. — A  magma  has  a  composition  differing  from 
that  of  an  igneous  rock  by  the  amount  of  material  in  the 
magma  which  escapes  prior  to  crystallization.  No 
analyses  are  available  showing  the  relative  amounts  of 
water  vapor  with  other  gases.  Iddings  believes  that 
99.9%  of  all  gases  escaping  from  magmas  consists  of 
water  vapor.  Other  gases  are:  CO,,  N,,  H,S,  0,  HC1, 
H_,,  SO,,  S,  CH4,  Fl,  B. 

The  more  basic  the  rock  is  the  greater  quantity  of 
gases  it  contains.  The  liberation  of  all  of  the  gases  in 
the  outer  seventy  miles  of  the  earth  would  double  the 
amount  of  nitrogen  and  carbon  dioxide  in  the  atmos- 
phere. Less  than  seventy  miles  of  earth's  crust  during 
consolidation  would  yield  all  of  the  gases  of  the  atmos- 
phere. Chamberlain  believes  that  the  gases  of  the  atmos- 
phere have  had  that  source.  The  same  conclusion  may 
be  drawn  with  regard  to  the  water  of  the  hydrosphere, 
assuming  that  the  average  per  cent  of  water  in  igneous 
rocks  is  2.3. 

Aids  in  the  Determination  of  Igneous  Rocks  in  Hand 
Specimens. — By  a  megascopic  examination  of  an  igneous 
rock  it  is  sometimes  possible  to  make  a  fairly  good  esti- 
mate as  to  what  the  rock  is.  Rocks  with  glassy  or  felsitic 
texture  may  easily  be  distinguished  from  granitoid  or 
porphyritic  rocks.  By  color  it  is  possible  to  determine 
whether  a  rock  is  acid  or  basic,  as  the  color  is  influenced 
by  the  amount  of  ferro-magnesian  minerals.  The  rocks 
which  are  known  to  occur  most  commonly  in  nature 
should  be  given  first  consideration  in  examining  the 
unknown  rock. 


134  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

The  texture  of  the  rock  should  be  examined  first.  Hav- 
ing noted  the  presence  or  absence  of  a  groundmass,  the 
feldspars  should  be  examined.  Pink  feldspar  is  usually 
orthoclase  or  microcline.  If  Carlsbad  twinning  can  be 
observed,  the  mineral  is  probably  orthoclase  or 
microcline. 

Plagioclase  feldspars  are  usually  white,  gray  or  bluish 
gray,  sometimes  with  a  flashing  blue  surface.  Albite 
twinning  is  often  observed  as  a  polysynthetic  striation. 
Labradorite  is  of  a  darker  blue  or  gray  than  the  other 
feldspars.  The  feldspars  are  less  transparent  and  glassy 
than  quartz. 

Quartz  is  recognized  by  its  vitreous,  fresh  appearance 
and  transparent  quality.  A  rock  containing  quartz  will 
contain  neither  leucite  nor  nephelite.  If  leucite  or  nephe- 
lite  can  be  determined,  quartz  is  absent.  This  fact  is 
inherent  in  the  chemical  composition  of  the  magma.  A 
rock  containing  leucite  or  nephelite  is  too  low  in  silica 
for  any  to  be  present  as  quartz  in  excess. 

Dark  minerals  which  appear  in  an  orthoclase-micro- 
cline  rock  are  biotite,  hornblende,  or  augite.  Biotite  is 
determined  by  the  flashing  black  surface  due  to  the  per- 
fect cleavage  plane.  A  knife  blade  may  be  used  to  test 
the  softness  and  the  ease  of  cleavage.  It  is  more  diffi- 
cult to  distinguish  hornblende  from  augite  in  that  they 
are  both  hard  and  not  readily  cleavable.  In  good  crystals, 
augite  shows  an  eight-sided  cross  section,  whereas  horn- 
blende has  a  six-sided  cross  section. 

The  black  minerals  of  a  plagioclase  rock  are  biotite 
and  hornblende  rather  than  augite.  If  the  rock  is  mainly 
basic,  has  a  dark  color,  and  a  dull,  stony  appearance,  the 
black  mineral  is  probably  augite.  Olivine  occurs  in  basic 
lavas  in  clear,  glassy  greenish-yellow  grains. 


GENERAL  DISCUSSION  OF  IGNEOUS  ROCKS  135 

A  dense,  volcanic  rock  which  shows  a  groundmass  and 
visible  quartz  is  either  rhyolite  or  dacite.  Without  fur- 
ther examination  the  observer  would  be  justified  to  call 
the  rock  rhyolite,  as  it  is  far  more  abundant  than  dacite. 

If  the  volcanic  rock  is  black  and  felsitic  or  stony  in 
appearance,  it  is  a  basalt.  If  the  rock  answers  neither 
of  these  descriptions  but  is  evidently  volcanic,  it  may  be 
a  trachyte,  a  phonolite,  or  an  andesite.  Of  the  three, 
andesite  is  the  most  probable,  as  it  is  the  most  common. 
It  usually  appears  medium  dark,  midway  between  the 
acid  and  basic  members  of  the  series.  If  Carlsbad  twin- 
ning is  seen  on  the  feldspar,  the  rock  may  be  trachyte 
instead  of  rhyolite.  If  leucite  is  distinguished,  it  is  a 
phonolite,  otherwise  there  would  be  no  justification  for 
naming  it  thus. 

A  rock  possessing  a  granitoid  texture  and  quartz  in 
some  abundance  may  be  called  a  granite  rather  than  the 
rarer  quartz  diorite.  If  it  contains  orthoclase  and  no 
quartz,  the  observer  would  doubtless  classify  the  rock 
either  as  a  syenite  or  as  a  nephelite  syenite,  although  the 
former  would  be  the  more  probable.  Diorites  are  darker 
than  the  syenites  and  may  be  inferred  from  this  fact 
alone  if  the  character  of  the  feldspars  cannot  be  deter- 
mined. If  plagioclase  can  be  distinguished  the  classifi- 
cation is  simplified.  The  latter  may  also  show  the  dark 
or  gray  blue  color  of  labradorite. 

Diabase  may  be  determined  by  a  peculiar  texture, 
commonly  called  diabasic  texture,  on  account  of  its  char- 
acteristic appearance.  White  plagioclase  is  intimately 
intergrown  with  augite  crystals,  the  plagioclase  having 
developed  first,  hence  taking  a  lath-shaped  texture.  The 
interstices  between  the  feldspars  were  later  filled  by  the 
augite.  It  is  dark  and  of  medium  grain. 


136  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

The  more  basic  rocks  are  determined  by  the  total 
absence  of  quartz  and  feldspar,  and  the  nature  of  the 
ferro-magnesian  mineral  comprising  the  greater  part  of 
the  rock. 


GENERAL  DISCUSSION  OF  IGNEOUS  ROCKS 


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138  OPTICAL  MINERALOGY  AND  PETROGRAPHY 


CHAPTER  9. 

IGNEOUS  ROCK  TYPES. 

Plutonic  Rocks. 
THE  GRANITE  FAMILY. 

Mineralogical  Composition. — Essential  minerals:  al- 
kali feldspar  and  quartz.  Common  minerals:  biotite, 
muscovite,  amphiboles,  pyroxenes.  Accessory  minerals : 
magnetite,  apatite,  zircon,  titanite,  garnet,  tourmaline. 

Texture. — Granitoid. 

Character. — Granites  are  generally  light  in  color,  in 
shades  of  white,  gray  and  pink,  occasionally  darker,  due 
to  an  increasing  amount  of  biotite,  amphiboles  or  pyrox- 
enes, in  which  case  the  rocks  are  liable  to  grade  into  the 
syenite  or  diorite  families. 

Microcline  is  more  common  in  granites  than  in  any 
other  kind  of  a  rock.  The  quartz  may  contain  minute 
rutile  needles  or  tiny  cavities  filled  with  gas  bubbles.  Bio- 
tite is  the  commonest  dark  silicate. 

Varieties  of  Granites. — There  are  two  varieties  of 
granites.  The  most  common  type  consists  of  the  alkali- 
lime  variety,  and  the  rarer  type  is  called  the  alkali-granite 
variety.  The  essential  difference  between  the  two  types 
is  that  the  alkali-lime  variety  grades  toward  and  into  the 
diorite  family,  and  the  alkali-granite  variety  grades 
toward  and  into  the  alkali  syenites  and  finally  into  the 
nephelite  syenites.  The  pyroxenes  and  amphiboles  of  the 
alkali-lime  variety  are  more  basic,  and  contain  consider- 


IGNEOUS  ROCK  TYPES  —  PLUTONIC  ROCKS     139 

able  amounts  of  magnesium  and  calcium.     These  min- 
erals do  not  appear  in  the  alkali  granites,  but  are  substi- 
tuted by  alkali  pyroxenes  and  amphiboles,  such  as  segi- 
rite  and  riebeckite. 
Classification : 

GRANITE  FAMILY. 

Alkali-Lime  Granites. — Containing  alkali  feldspar 
(orthoclase,  microcline,  albite,  perthite)  and  quartz. 

a.  Granitite,  with  addition  of  biotite. 

b.  Amphibole  granitite,  with  addition  of  biotite  and 
amphibole. 

c.  Pyroxene  granitite,  with  addition  of  biotite  and 
pyroxene. 

d.  Granite,  with  addition  of  muscovite  and  biotite. 

e.  Amphibole  granite,  with  addition  of  muscovite,  bio- 
tite and  pyroxene. 

f.  Pyroxene  granite,  with  addition  of  muscovite,  bio- 
tite and  pyroxene. 

g.  Tourmaline  granite,  with  addition  of  tourmaline. 

Alkali  Granites. — Containing  alkali  feldspar  and 
quartz. 

a.  Alkali  granitite,  with  addition  of  biotite. 

b.  Aegirite  granite,  with  addition  of  segirite. 

c.  Riebeckite  granite,  with  addition  of  riebeckite. 

d.  Aplite,  no  subordinate  mineral  except  possibly  a 
little  muscovite. 

BORDER  PHASES  OF  GRANITES. 

Pegmatites. — A  pegmatite  is  a  border  phase  of  a  gran- 
ite often  observed  on  the  edges  of  bosses  and  batholiths. 
They  are  usually  very  coarsely  crystalline  vein-granites, 
consisting  of  quartz,  feldspar,  muscovite,  tourmaline, 
beryl,  spodumene  and  others.  Due  to  the  immense  size 


140  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

which  the  crystals  attain,  pegmatites  are  sometimes 
called  "giant  granites."  The  largest  crystal  of  spod- 
umene  on  record  was  found  in  the  Etta  tin  mines  of  the 
Black  Hills.  This  crystal  measured  thirty  feet  in  height. 
Beryl  crystals  weighing  over  a  ton  have  been  recorded. 
Muscovite  mica  in  sheets  three  feet  in  diameter  are  quite 
common.  In  pegmatites,  the  essential  minerals  of  gran- 
ites are  not  always  present.  Quartz  and  beryl,  quartz  and 
tourmaline,  mica  and  quartz,  feldspar  and  tourmaline, 
are  all  possible  combinations. 

Pegmatites  are  usually  regarded  as  a  late  phase  of 
the  eruption  which  produced  the  granite.  The  common 
occurrence  of  such  minerals  as  tourmaline,  top'az  and 
apatite  in  pegmatite  leads  to  the  suggestion  that  the  influ- 
ence of  the  rare  elements  fluorine  and  boron  may  have 
had  some  influence  in  effecting  the  coarse  crystallization 
which  so  frequently  exists. 

Graphic  Granite. — Graphic  granite  is  a  variety  of  a 
pegmatite  which  consists  of  a  curious  form  of  inter- 
growth  of  quartz  and  the  feldspars  in  such  a  manner  that 
the  cross  fracture  of  the  vein  rock  exposes  a  cuneiform 
or  wedge-shaped  texture  resembling  the  writing  charac- 
ter of  the  ancient  Chaldeans  and  Assyrians.  The  most 
common  intergrowth  is  quartz  inclosed  in  orthoclase, 
microcline,  or  perthite.  Since  neither  of  the  minerals 
comprising  graphic  granite  possesses  any  definite  crystal 
shape,  it  is  evident  that  they  crystallized  from  solution 
at  the  same  time. 

Greisen. — Greisen  is  another  border  phase  of  a  gran- 
ite mass  which,  although  not  occurring  abundantly,  is  of 
economic  value  the  world  over  as  the  mother  rock  for 
cassiterite,  the  tin  ore.  It  is  a  granitoid  rock  composed 
of  quartz  and  muscovite,  or  some  related  white  mica,  as 
lepidolite  or  zinnwaldite. 


IGNEOUS  ROCK  TYPES  —  PLUTONIC  ROCKS     141 

Greisens  are  the  result  of  contact  action  on  gran- 
ites under  the  influence  of  mineralizers.  It  has  been 
suggested  that  the  water  and  fluorine  vapors  came  into 
contact  with  the  feldspars,  converting  them  to  micas. 

Orbicular  Granite. — Orbicular  granite  consists  of 
spherical  or  ellipsoidal  masses  of  basic  minerals  in  gran- 
ite. The  dark  minerals  are  commonly  biotite,  pyroxenes, 
or  amphiboles.  They  exist  in  the  nature  of  basic  segre- 
gations. 

Contact  Metamorphism. — Owing  to  the  amount  of 
heat  and  the  presence  of  mineralizers  which  are  given  off 
from  the  granitic  magma,  considerable  metamorphic 
effect  is  exerted  upon  adjacent  rocks  with,  which  the 
intruding  magma  comes  in  contact.  This  effect  may  not 
be  conspicuous  upon  igneous  and  previously  metamor- 
phosed rocks,  as  they  are  already  in  a  highly  crystalline 
condition.  The  effect  upon  sedimentary  rocks  is  usually 
considerable.  It  takes  the  form  of  a  more  or  less  com- 
plete recrystallization  of  the  secondary  and  undecom- 
posed  primary  minerals  composing  the  rocks.  In  addi- 
tion to  complete  recrystallization  there  is  quite  frequently 
an  addition  or  a  subtraction  of  constituents  or  an  ex- 
change of  constituents  with  the  magma. 

The  chief  effect  upon  sandstone  is  a  recrystallization 
of  the  rounded  grains  of  sand,  resulting  in  a  filling  of  the 
interstices  between  the  grains.  Quartzite  is  the  rock  de- 
veloped. If  the  sandstone  contained  considerable  clay  and 
other  impurities,  a  sericite  schist  would  develop. 

A  common  effect  of  an  intrusive  granite  in  contact 
with  a  limestone  formation  is  the  recrystallization  of  the 
limestone  to  a  marble.  Most  limestones  contain  consider- 
able percentages  of  argillaceous  and  siliceous  material, 
which  will  be  converted  to  lime  magnesium  silicates.  If 
there  is  no  opportunity  for  the  carbon  dioxide  of  the 


142  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

limestone  to  escape,  it  will  be  retained  in  the  marble  as 
calcite  or  dolomite.  If  there  is  an  opportunity  for  its 
release  through  fissures  or  joint  cracks,  the  resulting 
mass  may  consist  essentially  of  secondary  silicates,  some 
of  which  develop  by  a  recrystallization  of  the  original 
constituents  of  the  limestones  and  others  by  the  addition 
of  material  from  the  magma. 

The  contact  metamorphic  effect  of  an  intrusive  magma 
on  shale  is  pronounced  and  characteristic.  Immediately 
at  the  contact,  the  shale  is  converted  into  a  "hornfels" 
rock,  which  is  dense,  very  finely  crystalline,  extremely 
hard,  has  a  conchoidal  fracture  and  consists  chiefly  of 
quartz,  feldspar  and  biotite.  From  hornfels  to  the  unal- 
tered shale  the  following  stages  are  often  observed: 
highly  metamorphosed  mica  schist,  spotted  mica  slate, 
spotted  slate  lacking  the  conspicuous  development  of  the 
micas,  unaltered  shale.  This  change  may  be  almost  im- 
perceptible, and  may  extend  for  miles  from  the  actual 
contact.  The  chemical  composition  of  hornfels  and  shale 
are  often  very  similar,  although  at  times  it  shows  an  addi- 
tion of  material  in  the  hornfels. 

Economic  Uses  of  Granites. — Granite  is  more  exten- 
sively used  for  structural  purposes  than  any  other  igne- 
ous rock,  although  any  crystalline  rock  is  often  loosely 
called  granite  in  the  quarry  if  it  consists  of  silicates. 
Granite  is  the  strongest  of  the  common  building  stones, 
the  crushing  resistance  ranging  from  15,000  to  30,000 
pounds  per  square  inch  tested  on  two-inch  cubes. 

The  following  resistance  tests  show  the  average  in 
range : 

Granite  from  St.  Cloud,  Minn.,  26,250  to  28,000. 

Granite  from  Mystic  River,  Conn.,  18,125  to  22,250. 

Granite  from  Cape  Ann,  Mass.,  19,500. 

Granite  from  Vinal  Haven,  Maine,  25,700. 


IGNEOUS  ROCK  TYPES  —  PLUTONIC  ROCKS     143 

Upon  the  following  points  are  based  the  desirability 
of  granites  for  structural  purposes : 

1.  Homogeneity  of  texture. 

2.  Adaptability  to  tool  treatment. 

3.  Good  rectangular  jointing  in  the  quarry. 

4.  Pleasing  color. 

5.  Transportation  facilities. 

6.  Durability    as    affected    by    grain    and    mineral- 
content. 

A  light  color  is  generally  more  desirable  than  a  dark 
one,  and  a  medium  grain  is  more  favorable  for  durability 
than  a  coarse  grain.  The  Rapakiwi  granite  of  southern 
Finland  is  used  freely  in  Petrograd  for  columns.  It  con- 
tains large  red  orthoclase  crystals,  which  give  the  rock 
a  prevailing  red  color,  greenish  plagioclase,  smoky  quartz 
and  biotite.  The  disintegration  is  found  to  be  rapid,  as 
the  jointing  or  fracture  occasioned  by  the  cleavage  planes 
of  one  mineral  tends  to  continue  into  the  others. 

Relationship. — Granite  approaches  syenite  by  insensi- 
ble gradations  with  decrease  in  quartz.  It  approaches 
diorite  with  increase  in  hornblende  or  biotite  and  plagio- 
clase. Intermediate  varieties  are  called  granodiorites. 
With  increase  in  augite  and  plagioclase,  granite  ap- 
proaches gabbro. 

Geographical  Distribution. — Granite  occurs  abun- 
dantly along  the  Atlantic  Coast  from  Virginia  into  Can- 
ada. It  is  extensively  quarried.  The  Quincy  granite 
from  Quincy,  Mass.,  is  a  well-known  building  granite. 
In  Minnesota,  Wisconsin,  and  northern  Michigan,  and 
northward  in  Ontario,  much  of  the  Pre-Cambrian  crys- 
talline area  known  geologically  as  the  Laurentian  High- 
land is  granite.  It  is  found  widespread  in  the  West, 
existing  in  the  Black  Hills,  in  the  Wasatch,  the  Rocky 
and  the  Sierra  Mountains. 


1 

2 

3 

4 

5 

6 

7 

73.23 

77.50 

69.00 

67.70 

61.90 

69.46 

66.84 

.  15.47 

10.10 

14.80 

14.80 

13.20 

17.50 

18.32 

.... 

2.30 

2.10 

3.60 

2.30 

2.27 

.    3.34 

2.70 

.90 

3.40 

2.30 

.... 

.20 

.24 

.60 

1.10 

1.60 

4.60 

.30 

.81 

.      .80 

2.30 

3.80 

3.90 

3.50 

2.70 

3.31 

1.70 

3.20 

2.50 

4.10 

2.70 

2.93 

5.14 

.    4.38 

4.00 

4.50 

4.30 

6.10 

4.07 

2.80 

.65 

.3 

.7 

1.00 

1.10 

.82 

.46 

.  99.81 

100.70 

99.60 

102.00 

99.00 

99.95 

100.49 

2.68 

2.62 

2.72 

2.68 

.... 

144  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

'V 

Analyses  of  Granites : 

SiO2     . 
A1203 
Fe,0, 
FeO 
MgO    . 
CaO 
Na2O   . 
K2O 
H,O     . 

Total 
Sp.  Gr. 

1.  Granite  from  Carlsbad,  Bohemia,  Austria. 

2.  Granite  from  Baveno,  Lake  Maggiore,  Italy. 

3.  Granite  from  Barr,  Lower  Alsace,  Germany. 

4.  Amphibole  granitite  from  Barthoga,  Sweden. 

5.  Pyroxene  granite  from  Lavellme,  Vosges  Mountains,  France. 

6.  Alkali  granite  from  Chester,  Massachusetts. 

7.  Augite  soda  granite  from  Kekequabic  Lake,  Minnesota. 

Discussion  of  Analyses: 

1.  Granite  contains  more  silica  than  any  other  plu- 
tonic  rock. 

2.  Alumina  content  is  not  as  high  as  in  the  syenites. 
It  is  present  chiefly  in  feldspars  and  biotite. 

3.  Iron  content  is  generally  low.    It  is  present  chiefly 
in  biotite,  amphiboles,  pyroxenes,  and  possibly  magnetite. 

4.  Magnesia  content  is  low,  indicating  an  absence  of 
many  ferro-magnesian  minerals. 

5.  Lime  content  is  low.     It  is  present  chiefly  in  a 
few  acid  plagioclases,  in  amphiboles  and  pyroxenes. 

6.  Potash    predominates    over    soda,    occurring    in 
alkali  feldspar  and  biotite. 

7.  Soda  rarely  predominates  over  potash.     When  it 
does   (see  Analysis  7),  albite  is  the  chief  feldspar.     It 
marks  a  gradation  toward  the  diorites. 


IGNEOUS  ROCK   TYPES  —  PLUTONIC  ROCKS  145 

8.  A  high  water  content  is  probably  due  to  the 
formation  of  secondary  minerals,  as  serpentine,  kaolinite, 
etc.    Water  in  small  quantities  exists  in  many  primary 
minerals:    micas,  amphiboles. 

9.  The  presence  of  apatite  as  an  accessory  mineral 
is  indicated  by  the  presence  of  P20-,. 

10.  The    darker   granites   have   the   higher   specific 
gravities. 

11.  Granite  is  similar  to  rhyolite  in  composition. 

12.  The  alkali-lime  granites  contain  more  iron,  mag- 
nesia, and  lime  than  do  the  alkali  granites.  The  alkali 
granites  are  richer  in  soda,  potash,  and  possibly  silica. 

THE  SYENITE  FAMILY. 

Mineralogical  Composition. — Alkali  feldspar  with 
little  or  no  quartz. 

Texture. — Granitoid  with  groundmass  absent.  Some- 
times porphyritic. 

Character  and  Distribution. — Syenites  are  allied  with 
nephelite  syenites,  into  which  they  grade  with  increase 
of  soda.  They  merge  into  the  diorites  with  increase  in 
plagioclase.  Their  geological  occurrence  is  practically 
the  same  as  that  of  granites.  They  are  often  found  at 
the  rims  of  granite  bosses  or  batholiths  where  there  has 
been  a  decrease  in  silica  in  the  form  of  quartz. 

Geographically  the  syenites  form  the  basement  of 
the  White  Mountains,  and  occur  in  dikes  near  Little 
Rock,  Arkansas.  Many  minor  occurrences  have  been 
recorded.  VARIETIES. 

Alkali-Lime  Syenite. — Essential  minerals  are  alkali 
feldspar,  with  a  little  basic  plagioclase. 

a.  Amphibole  syenite,  with  addition  of  amphibole. 

b.  Mica  syenite,  with  addition  of  mica. 

c.  Pyroxene  syenite,  with  addition  of  pyroxene. 


146 


OPTICAL  MINERALOGY  AND  PETROGRAPHY 


Alkali  Syenite. — The  alkali  syenites  are  rare.  The 
same  three  types  occur  in  this  group  as  occur  in  the  alkali- 
lime  group,  except  that  the  dark  minerals  are  alkali 
pyroxenes  and  amphiboles. 

Belonging  to  the  alkali-lime  group  is  a  rock  called  a 
"monzonite,"  which  grades  over  into  the  diorites,  as  it 
contains  both  alkali  feldspar  and  plagioclase.  A  rock 
associated  with  the  copper  ore  deposits  of  Butte,  Mon- 
tana, is  a  more  acid  rock  of  this  type,  called  a  quartz 
monzonite.  It  covers  an  area  seventy  miles  by  forty,  and 
is  known  as  the  Bowlder  Batholith. 

The  corundum  syenites  north  of  Kingston,  Ontario, 
are  alkali  syenites  composed  of  pink  orthoclase.  and  a 
greenish  corundum  which  is  used  as  an  abrasive. 

The  colors  of  the  syenites  are  light,  although  usually 
darker  than  the  granites.  The  crushing  strength  is 
greater. 

Analyses  of  Syenites: 

Si02    . 
TiO2      . 

Fe2O3     . 
FeO    . 
MgO      . 
CaO    . 
Na:0     . 

H20 
P20S    . 

Total     . 
Sp.  Gr. 

1.  Mica  syenite  (alkali  type)  from  Tonsenoos,  near  Christiania, 

Norway. 

2.  Mica  syenite    (alkali  lime  type)    from  Gangenbach,   Black 

Forest,  Germany. 


1 

2 

3 

4 

5 

6 

64.00 

51.00 

59.40 

52.88 

59.78 

59.83 

1.80 

.30 

17.40 

14.50 

17.90 

20.30 

16.86 

16.85 

.    1.00 

4.20 

2.00 

3.63 

3.08 



2.30 

4.40 

6.80 

2.58 

3.72 

7.01 

.      .60 

8.20 

1.80 

.79 

.69 

2.61 

1.00 

5.10 

4.20 

3.03 

2.96 

4.43 

.    6.70 

1.80 

1.20 

5.73 

5.39 

2.44 

6.10 

7.20 

6.70 

4.50 

5.01 

6.57 

.     1.20 

1.00 

.40 

1.01 

1.58 

1.29 

.70 

.60 

.54 

101.40 

99.90 

101.30 

100.99 

99.07 

101.03 



2.77 

2.73 

2.67 

2.73 

IGNEOUS  ROCK  TYPES  —  PLUTONIC  ROCKS     147 

3.  Hornblende  syenite  from  Biella,  Piedmont,  Italy. 

4.  Augite  syenite  from  Byskoven,  near  Laurvik,  Norway. 

5.  Syenite  from  Custer  County,  Colorado. 

6.  Hornblende  syenite  from  Plauenschen  Grund,  near  Dresden. 

Discussion  of  Analyses: 

1.  Silica  content  is  lower  than  in  granites,  due  to  de- 
crease of  quartz. 

2.  Alumina  content  is  higher  than  in  granites,  due  to 
relative  increase  in  feldspars. 

3.  Iron,  magnesia  and  lime  contents  are  higher,  due 
to  increase  in  ferro-magnesian  minerals,  chiefly  horn- 
blende. 

4.  Alkali  content  is  higher,  due  to  increase  of  felds- 
spars. 

5.  The  high  water  content  is  due  to  hydration  of  the 
secondary  minerals. 

6.  The  specific  gravity  is  higher  in  that  of  granites, 
due  to  the  increase  in  ferro-magnesian  minerals. 

7.  The  alkali-lime  syenites  contain  more  lime  and 
magnesia  than  the  alkali  syenites. 

NEPHELITE  AND  LEUCITE  SYENITES. 

Mineralogical  Composition. — Alkali  feldspar  and 
nephelite  or  leucite. 

Texture. — Granitoid,  sometimes  porphyritic. 

Character  and  Distribution. — Nephelite  and  leucite 
syenites  are  white  to  smoky  gray  in  color,  and  contain 
very  few  accessory  minerals.  When  present,  they  usually 
are  biotite,  segirite,  and  an  alkali  amphibole  called  bar- 
kevikite. 

These  types  are  comparatively  rare,  occurring  espe- 
cially as  dikes.  They  are  known  in  North  America  at 


148  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

Montreal  and  Dungammon  (Ontario),  Litchfield 
(Maine),  Red  Hill  (New  Hampshire),  Salem  (Massa- 
chusetts), Beemersville  (New  Jersey),  and  near  Little 
Rock  (Arkansas),  in  well-known  exposures,  though  they 
have  a  widespread  occurrence. 

Of  economic  importance  is  the  occurrence  of  rare  min- 
eral containing  zirconium,  tantalum,  titanium,  yttrium, 
cerium,  lanthanum,  terbium,  and  other  rare  elements.  In 
the  nephelite-syenite  pegmatites  of  southern  Norway 
about  800  of  these  rare  minerals  have  been  recorded.  A 
corundiferous  nephelite  syenite  is  found  in  commercial 
quantities  in  Canada. 

Analyses  of  Nephelite  and  Leucite  Syenites: 


1 

2 

3 

4 

SiO,     . 

56.30 

50.36 

60.39 

50.90 

AUO, 

.    24.14 

19.34 

22.51 

19.67 

Fe2O,    . 

1.99 

6.94 

.42 

7.76 

FeO 

2.26 

MgO     . 

.13 

.... 

.13 

.36 

CaO 

69 

3.43 

.32 

4.38 

Na,O    . 

9.28 

7.64 

8.44 

4.38 

K20 

.          .          .      6.79 

7.17 

4.77 

6.77 

H2O      . 

1.58 

3.51 

.57 

1.38 

Total 100.90         98.39         99.81       100.01 

1.  Nephelite  syenite  from  Ditro,  Transylvania,  Hungary. 

2.  Nephelite  syenite  from  Beemersville,  Sussex  County,  New 

Jersey. 

3.  Nephelite  syenite  from  Litchfield,  Maine. 

4.  Leucite  syenite  from  Magnet  Cove,  Arkansas. 

Discussion  of  Analyses: 

1.  Silica  content  is  lower  than  in  the  syenites,  as  neph- 
elite has  44%.  silica,  and  the  minerals  which  it  replaces 
have  several  per  cent  more. 


IGNEOUS  ROCK   TYPES  —  PLUTONIC  ROCKS  149 

2.  Alumina  content  is  higher  than  in  any  other  plu- 
tonic  rock,  due  to  the  presence  of  nephelite  or  leucite. 

3.  Iron  content  is  variable,  but  magnesia  and  lime 
contents  are  lower  than  in  the  syenites,  due  to  the  absence 
of  many  ferro-magnesian  minerals. 

4.  Alkali  content  is  higher  than  in  any  other  plutonic 
rock.     Soda  predominates  over  potash  in  the  nephelite 
syenites.     In  the  leucite  syenites  potash  increases,  but 
may   not  predominate,   as   leucite   readily   decomposes, 
allowing  the  potash  to  be  removed. 

5.  The  specific  gravity  is  less  than  that  of  the  syenites. 

DIORITE  FAMILY. 

Mineralogical  Composition. — Acid  plagioclase  with  or 
without  quartz,  and  some  dark  mineral,  most  commonly 
an  amphibole  near  green  hornblende.  Biotite  is  common. 

Texture. — Granitoid,  at  times  porphyritic. 

Character. — The  color  of  diorite  is  dark,  due  to  the 
ferro-magnesian  minerals.  It  grades  from  the  alkali-lime 
granite  type  by  decrease  in  alkali  feldspar  and  increase 
in  acid  plagioclase.  Certain  intermediate  phases  are 
called  granodiorites,  containing  both  alkali  feldspar  and 
acid  plagioclase. 

Diorites  are  not  very  common  in  North  America. 
They  occasionally  form  on  the  edge  of  granite  bosses  or 
batholiths. 

CLASSIFICATION  OF  THE  DIORITES. 
With  Quartz  Without  Quartz 

Quartz  mica  diorite.  Mica  diorite. 

Quartz  hornblende  diorite.      Hornblende  diorite. 
Quartz  augite  diorite.  Augite  diorite. 

Quartz  hypersthene  diorite.     Hypersthene  diorite. 


150 


OPTICAL  MINERALOGY  AND  PETROGRAPHY 


Analyses  of  Diorites: 

1 

2 

3 

4 

5 

Si02 

61.22 

64.12 

56.09 

52.45 

52.00 

A1203 

.    16.14 

16.50 

16.03 

18.63 

15.75 

5'e20. 

3.01 

2.71 

3.12 

11.40 

3.55 

FeO    . 

.      2.58 

4.26 

4.77 

1.19 

12.84 

MgO       . 

4.21 

2.34 

8.03 

5.16 

3.42 

CaO    . 

.      5.46 

4.76 

6.73 

6.84 

7.39 

NajO       . 

4.48 

3.92 

3.49 

2.64 

2.37 

K2O    . 

.      1.87 

1.92 

1.87 

.37 

1.24 

H20 

.44 

.73 

.16 

2.40 

.35 

Total 


99.41       101.26 


100.13       100.82         99.91 
Electric  Peak,  Yellow- 


1.  Pyroxene  amphibole  biotite  diorite. 

stone  Park. 

2.  Quartz  mica  hypersthene  diorite  from  Pfundersberg,  Tyrol. 

3.  Mica  hypersthene  diorite  from  Campomaior,  Portugal. 

4.  Amphibole  diorite  from  Neunseestein  Barr,  Alsace. 

5.  Augite  diorite  from  Richmond,  Minnesota. 

Discussion  of  Analyses: 

These  analyses  when  compared  with  the  analyses  of 
granite  show  that — 

1.  Iron,   lime,  magnesia  and  alumina  contents   are 
higher. 

2.  Alkali  content  is  lower. 

3.  Soda  predominates  over  potash. 

4.  Silica  is  lower,  due  to  change  of  feldspar. 

5.  The  specific  gravity  is  higher. 

GABBRO  AND  NORITE  FAMILY. 

Mineralogical    Composition. — Basic    plagioclase    and 
usually  a  pyroxene. 

Texture. — Granitoid.    Never  porphyritic. 
Relationship. — Gabbros  grade  by  decrease  in  plagio- 
clase to  the  more  basic  pyroxenites  and  peridotites. 


IGNEOUS  ROCK  TYPES  —  PLUTONIC  ROCKS     151 

Varieties. — Essential  to  all,  basic  plagioclase. 

1.  Gabbro,  with  addition  of  diallage. 

2.  Hornblende  gabbro,  with  addition  of  hornblende. 

3.  Olivine    gabbro,    with    addition    of    olivine    and 
diallage. 

4.  Norite,  with  addition  of  hypersthene,  bronzite  or 
enstatite. 

5.  Olivine  norite,  with  addition  of  hypersthene,  bron- 
zite or  enstatite  and  olivine. 

6.  Anorthosite,  composed  chiefly  of  labradorite.     It 
may  contain  a  few  dark  minerals  which,  when  metamor- 
phosed, cause  the  development  of  almandite  garnets  in 
considerable  quantities. 

Concentration  of  Magnetite. — The  concentration  of 
magnetite  in  many  gabbroid  magmas  took  place  during 
the  process  of  solidification  along  the  lower  border  of 
the  magma.  This  concentration  was  effected  by  the  early 
crystallization  of  the  magnetite  from  solution,  its  high 
specific  gravity,  convection  currents,  etc.  Magnetite  of 
this  occurrence  has  been  found  in  commercial  quantities 
in  the  Adirondacks  and  in  Lake  and  Cook  counties  of 
northern  Minnesota.  It  is  usually  titaniferous,  due  to 
an  intimate  association  with  the  mineral  ilmenite. 

Nickeliferous  Pyrrhotite. — In  the  Sudbury  district  of 
Ontario,  great  quantities  of  nickeliferous  pyrrhotite  and 
workable  amounts  of  chalcopyrite  are  found  in  the  norite. 
They  occur  as  magmatic  segregations.  The  pyrrhotite 
is  an  important  source  of  nickel. 

In  Lancaster  County,  Pennsylvania,  nickeliferous 
pyrrhotite  is  observed  along  the  contact  of  a  metamor- 
phosed basic  igneous  rock  called  amphibolite. 

Platinum  is  believed  to  occur  minutely  disseminated 
in  rocks  of  this  type,  the  weathering  of  which  has  sftp- 
plied  the  placer  deposits. 


152 


OPTICAL  MINERALOGY  AND  PETROGRAPHY 


Analyses  of  Gabbros  and  Norites: 


SiO,, 

AW, 

Fe2O; 

FeO 

MgO 

CaO 


1 

2 

3 

4 

5 

54.47 

44.10 

46.70 

49.10 

49.95 

26.45 

24.50 

22.20 

21.90 

19.17 

1.30 

7.90 

.80 

6.60 

4.72 

.67 

6.50 

5.50 

4.50 

6.71 

.69 

3.80 

10.30 

3.00 

5.03 

10.80 

12.00 

11.70 

8.20 

9.61 

4.37 

1.70 

1.70 

3.80 

3.13 

.92 

.20 

.10 

1.60 

.74 

.53 

.60 

1.10 

1.90 

.09 

K2O 
H2O 


Total  .          .          .  100.20       101.30       100.10       100.89         99.84 

Sp.  Gr.   .          .          .  2.72  3.04  3.02  2.94  2.94 

1.  Anorthosite  from  Adirondacks,  New  York. 

2.  Gabbro  from  Mount  Hope,  Baltimore,  Maryland. 

3.  Olivine  gabbro  from  Langenlois,  Lower  Austria. 

4.  Hornblende  gabbro  from  Duluth,  Minnesota. 

5.  Norite  from  Mpnsino,  near  lorea,  Piedmont,  Italy. 

Discussion  of  Analyses. — The  analyses  compared  with 
analyses  of  diorites  show : 

1.  A  lower  silica  content  due  to  the  absence  of  quartz 
and  the  decreasing  basicity  of  the  feldspars. 

2.  Higher  alumina,  lime,  iron  and  magnesia  content. 
High  magnesia,  as  in  Analysis  4,  suggests  olivine. 

3.  Lower  alkali  content. 

4.  Higher  specific  gravity. 

ESSEXITE  FAMILY. 

Mineralogical  Composition. — Basic  plagioclase,  with 
varying  amounts  of  subordinate  orthoclase.  Nephelite 
or  sodalite  may  be  present.  The  dark  minerals  are  au- 
gite,  biotite,  and  a  brown  amphibole  called  barkevikite. 
Olivine  and  apatite  sometimes  occur.  The  plagioclase 
is»usually  labradorite,  rarely  andesine. 


IGNEOUS  ROCK  TYPES  —  PLUTONIC  ROCKS     153 

Relationship. — Essexite  is  related  to  the  gabbros  much 
as  monzonite  is  related  to  the  syenites.  It  was  originally 
classed  with  the  gabbros,  but  is  more  generally  associated 
with  the  alkali  and  nephelite  syenites.  It  may  be  con- 
sidered intermediate  between  this  type  and  the  gabbroid 
type.  It  was  first  recognized  in  association  with  neph- 
elite syenites  near  Boston. 

Discussion  of  Analyses  (See  next  table)  : 

1.  Low  silica  content. 

2.  High  alumina  and  iron  content. 

3.  Equal  amounts  of  lime  and  the  alkalies. 

4.  Soda  predominates  over  potash. 

5.  Magnesia  content  low  as  compared  with  gabbro. 

6.  P2O5  high  due  to  apatite. 

THERALITE-SHONKINITE-MALIGNITE  FAMILY. 
Mineralogical  Composition. — Basic  plagioclase,  neph- 
elite or  leucite,  pyroxene  with  some  biotite,  and  rarely 
amphibole.     Members  of  the  sodalite  group  may  accom- 
pany the  nephelite. 

Relationship. — These  rocks  are  related  to  essexite, 
and  grade  into  them.  The  presence  of  nephelite  or  leu- 
cite  is  the  essential  difference.  Chemically  they  are  quite 
similar. 

Distribution. — Theralite  was  first  found  in  the  Crazy 
Mountains,  Montana,  near  Livingston,  and  described  by 
J.  E.  Wood,  of  Harvard. 

Shonkinite  was  described  by  Weed  and  Pirrson,  from 
Square  Butte  in  the  Highwood  Mountains  of  Montana,  as 
a  border  phase  of  a  sodalite  syenite  laccolith.  It  contains 
little  nephelite,  but  has  instead  sanadine.  Consequently, 
potash  prediminates  over  soda.  In  theralite,  soda 
predominates. 


154  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

Malignite  was  named  by  Lawson,  from  Puba  Lake, 
Ontario.  It  contains  chiefly  segirite,  augite,  biotite,  ortho- 
clase,  nephelite  and  titanite. 

Discussion  of  Analyses: 

1.  Chemical  resemblance  to  essexite. 

2.  Low  silica. 

3.  Equal  lime  and  magnesia  content. 

4.  Magnesia  higher  than  in  essexite. 


Analyses : 


SiO2 
A12(X 


FeO 

MgO 

CaO 

Na20 

K20 

H20 


1 

2 

3 

4 

5 

6 

47.94 

43.17 

46.73 

47.85 

42.79 

46.06 

17.44 

15.24 

10.05 

13.24 

21.59 

10.74 

6.84 

7.61 

3.53 

2.74 

4.39 

3.17 

6.51 

2.67 

8.20 

2.65 

2.33 

5.61 

2.02 

5.81 

4.68 

5.68 

1.87 

14.74 

7.47 

10.63 

13.22 

14.36 

11.76 

10.55 

5.63 

5.68 

1.81 

3.72 

9.31. 

1.31 

2.79 

4.07 

3.76 

5.25 

1.67 

5.14 

2.04 

3.57 

1.24 

2.74 

.99 

1.44 

1.04 

.... 

1.51 

2.42 

1.70 

.21 

Total        .      .     99.02         98.45         99.73       100.65         98.40         98.97 

1.  Essexite  from  Salem  Rock,  near  Boston. 

2.  Theralite  from  Martinsdale,  Crazy  Mountains,  Montana. 

3.  Shonkinite  from  Square  Butte,  Highwood  Mountains,  Mon- 

tana. 

4.  Malignite  from  Puba  Lake,  Rainy  Lake  District,  Ontario. 

5.  Ijolite  from  Iwaara,  Finland. 

6.  Missourite  from  Shonkin  Creek,  Highwood  Mountains,  Mon- 

tana. 

IJOLITE  AND  MISSOURITE. 

Mineralogical  Composition. — Ijolite  contains  aegirite- 
augite  and  nephelite,  often  with  apatite,  titanite,  and 
andradite  as  accessories.  It  is  nonfeldspathic. 


IGNEOUS  ROCK  TYPES  —  PLUTONIC  ROCKS     155 

Missourite  contains  augite,  leucite,  olivine,  and  biotite 
with  accessories.  It  is  nonfeldspathic. 

Relationship. — These  rocks  are  end  products  of  the 
series  beginning  with  essexite.  They  are  closely  related 
to  the  theralite-shonkinite  rocks  and  are  distinguished 
from  them  by  the  fact  that  they  contain  no  feldspars. 
Ijolite  was  first  found  on  Mount  Iwaara  in  northern 
Finland. 

PERIDOTITE  FAMILY. 

Mineralogical  Composition. — Olivine  with  pyroxenes, 
amphiboles,  or  biotite.  No  feldspars  are  present. 

Relationship. — The  peridotites  grade  from  olivine 
gabbros  by  the  elimination  of  the  feldspars.  They  are 
found  on  the  edges  of  gabbro  and  norite  bosses.  They 
are  regarded  as  ultra  basic. 

Classification. — Olivine  is  essential  in  all  varieties. 

1.  Sherzolite,  by  addition  of  diopside  and  enstatite. 

2.  Harburgite,  by  addition  of  enstatite. 

3.  Wehrlite,  by  addition  of  diallage  and  hornblende. 

4.  Cortlandite  by  addition  of  hornblende. 

5.  Dunite  chiefly  olivine.    It  may  contain  chromite  or 
chrome  spinel. 

6.  Kimberlite,  which  was  named  from  its  occurrence 
in  Kimberley,  South  Africa,  is  a  peridotite  found  in  the 
truncated  cones  of  extinct  volcanoes.     In  its  type  local- 
ity it  weathers  to  a  soft  serpentine  rock  called  "blue 
ground."    It  is  the  mother  rock  of  the  diamond.    A  sim- 
ilar rock  has  been  found  in  southern  Arkansas,  where 
diamonds  are  likewise  found  in  commercial  quantities. 
Small  diamonds  have  been  found  in  a  peridotite  rock  in 
Elliot  County,  Kentucky. 

Garnierite,  the  chief  ore  of  nickel,  is  a  secondary  min- 
eral associated  with  serpentinized  peridotite,  probably  as 


156 


OPTICAL  MINERALOGY  AND  PETROGRAPHY 


an  alteration  of  a  nickel-bearing  olivine.     The  French 
locality  of  New  Caledonia  is  the  only  important  locality. 

Analyses : 


1 

2 

3 

4 

SiO. 

.                   .         41.44 

34.98 

53.98 

44.01 

ALOs 

.      6.63 

10.80 

1.32 

11.76 

FeoOt 

13.87 

1.42 

1.41 

15.01 

FeO 

.      6.30 

21.33 

3.90 

MgO     . 

18.42 

19.30 

22.59 

25.25 

CaO 

.      7.20 

.43 

15.49 

4.06 

Na20    . 

.24 

.17 

.... 

K20 

.         .        .93 

5.42 

H20      . 

5.60 

1.28 

.83 



100.63 


95.13 
3.276 


99.59 
3.301 


100.09 


Total         .... 
Sp.   Gr. 

1.  Amphibole  peridotite  from  Sebreizheim,  Baden,  Germany. 

2.  Mica  peridotite  from  Kaltes  Thai,  Harzburg,  Germany. 

3.  Pyroxenite  from  Baltimore,  Maryland. 

4.  Pyroxenite  from  Meadowbrook,  Montana. 

Discussion  of  Analyses: 

1.  Lower  silica  and  alumina  content  than  in  gabbro, 
due  to  the  absence  of  feldspars. 

2.  Iron  content  varies,  depending  upon  the  dark  min- 
eral present. 

3.  Magnesia  content  higher  than  in  any  other  normal 
plutonic  rock. 

4.  Lime  content  varies. 

5.  Alkali  content  less  than  that  of  any  other  igneous 
rock.     In  a  mica  peridotite,  potash  predominates  over 
soda,  an  unusual  case  among  alkali-lime  rocks. 

6.  Specific  gravity  highest  of  the  normal  plutonic 
rocks. 


IGNEOUS  ROCK   TYPES  —  PLUTONIC  ROCKS  157 

PYROXENITE  AND  HORNBLENDITE  FAMILY. 

Mineralogical  Composition. — These  rocks  consist  of  a 
single  pyroxene,  or  a  single  amphibole,  or  two  or  more 
minerals  of  the  same  group. 

Relationship. — They  are  the  end  products  of  the  ultra 
basic  rocks  grading  from  the  peridotites  by  the  elimina- 
tion of  the  olivine.  The  varieties  depend  upon  the  min- 
eral which  composes  the  rock,  the  rock  name  usually 
being  the  mineral  name  with  the  suffix  "ite"  added  to  it. 

Varieties  of  the  family  are  diallagite,  enstatite,  bronz- 
itite,  hypersthenite,  hornblendite. 

Occurrence. — The  pyroxenites  are  usually  found  in 
association  with  gabbro  and  norite  masses.  Peridotites 
may  have  a  similar  occurrence.  The  serpentine  deposits 
of  Quebec  and  New  England  occur  in  this  association. 
Serpentine  asbestos  is  extracted  in  commercial  quantities. 


158  OPTICAL  MINERALOGY  AND  PETROGRAPHY 


CHAPTER  10. 

IGNEOUS  ROCK  TYPES. 

Volcanic  Rocks. 
THE  RHYOLITE  FAMILY. 

Mineralogical  Composition. — Orthoclase,  oligoclase, 
quartz.  Biotite,  hornblende.  The  rhyolites  are  chem- 
ically the  equivalents  of  the  granites,  particularly  the 
alkali-lime  type. 

Texture. — Well  developed  groundmass,  often  largely 
glassy.  Frequently  porphyritic. 

Relationship. — Rhyolite  grades  imperceptibly  into 
trachyte,  granite,  and  dacite.  Unless  quartz  is  recog- 
nized, a  microscopic  examination  is  necessary  to  dis- 
tinguish rhyolite.  It  may  easily  be  confused  with  dacite 
unless  the  polysynthetic  twinning  of  the  feldspar  char- 
acteristic of  dacite  can  be  seen. 

Character. — The  term  "rhyolite"  comes  from  the 
Greek  verb  rhein,  "to  flow,"  because  of  the  flow  structure 
frequently  observed.  Liparite  is  a  synonymous  term 
used  largely  in  Europe.  It  was  named  from  the  Lipari 
Islands,  in  Sicily.  Quartz  porphyry  is  a  term  often 
applied  to  the  rhyolites  which  have  crystallized  as 
intruded  sheets,  laccoliths,  dikes,  and  sills.  The  glassy 
portion  is  characterized  by  its  behavior  between  crossed 
nicols,  remaining  dark  during  a  complete  revolution  of 
the  stage. 


IGNEOUS  ROCK  TYPES  —  VOLCANIC  ROCKS     159 

The  processes  of  weathering  of  the  rhyolites  are  the 
same  as  take  place  in  granites,  ordinary  decomposition  by 
atmospheric  agencies  giving  rise  to  the  formation  of  the 
hydrous  aluminum  silicates.  Metamorphic  processes 
develop  schistose  textures  leading  in  extreme  cases  to 
the  development  of  sericite  schists. 

Early  in  the  study  of  volcanic  rocks  it  was  customary 
to  distinguish  two  types  —  those  which  had  erupted  pre- 
vious to  Tertiary  times,  and  those  which  had  erupted  after 
Tertiary  times.  The  former  were  called  Paleovolcanic, 
and  the  latter  were  called  Neovolcanic.  Fortunately,  this 
classification  did  not  survive. 

Classification. — Rhyolites  are  regarded  by  some  writ- 
ers as  porphyritic  rocks  with  phenocrysts  of  quartz  and 
alkali  feldspars  in  a  groundmass  which  is  wholly  glassy 
or  a  very  finely  crystalline  aggregate  of  quartz  and  feld- 
spar. They  classify  in  the  "glasses"  all  varieties  of  vol- 
canic rocks  in  which  chilling  has  prevented  crys- 
tallization. 

The  classification  here  adopted  combines  the  glasses 
with  the  rhyolites. 

Volcanic  glasses  are  obsidian,  pumice,  pitchstone,  and 
perlite. 

OBSIDIAN  is  a  dense,  homogeneous  glass  with  a  low 
percentage  of  water. 

PUMICE  is  a  cellular  glass  formed  by  the  expansion 
of  the  cooling  magma  by  the  escaping  steam  bubbles.  It 
is  light,  very  porous,  and  may  resemble  blast-furnace 
slag. 

PITCHSTONE  is  essentially  the  same  as  obsidian,  with 
a  higher  percentage  of  water.  It  is  more  resinous  in 
appearance,  giving  it  a  greasy  or  pitchy  luster. 


160 


OPTICAL  MINERALOGY  AND  PETROGRAPHY 


PERLITE  is  a  pitchstone  which  has  a  spheroidal 
arrangement  of  the  particles,  giving  rise  to  a  rounded 
fracture. 

Pantellerite. — Pantellerite  is  a  volcanic  rock  corre- 
sponding to  the  alkali  granites.  It  is  rare,  and  occurs 
so  far  as  known  only  on  the  island  of  Pantellerea,  in  the 
Mediterranean  Sea.  It  contains  a  rare  feldspar  called 
anorthoclase,  which  is  an  isomorphous  mixture  of  albite 
and  orthoclase. 

Distribution. — Rhyolites  are  widespread  throughout 
the  Western  States.  Obsidian  Cliff  in  Yellowstone  Park, 
Silver  Cliff  in  Utah,  extinct  volcanoes  in  New  Mexico, 
Utah,  Montana  and  California  (Mono  Lake),  are  well- 
known  examples.  In  Leadville,  Colorado,  they  are  asso- 
ciated with  the  ore  deposits. 

Along  the  Eastern  Coast,  remnants  of  rhyolite  lavas 
from  ancient  Pre-Cambrian  volcanoes  have  been  found 
in  New  Brunswick,  Maine,  Massachusetts,  and  Pennsyl- 
vania. 


Analyses : 


Si02 


Fe.0,    . 
FeO 
MgO     . 
CaO 
NazO    . 

H,0      . 

Total 
Sp.  Gr. 


123 

83.59         77.00         75.60 

5.42         12.80         11.50 

1.90  2.40 


30 

3.44           1.40  .80 

5.33           3.00  2.90 

1.37           4.10  5.90 

.76             .70  1.00 


4 

68.30 

10.90 

3.70 

.40 

.20 

1.40 

7.10 

4.10 


99.91       101.20       100.10       101.10 
2.54  2.41  2.44  2.48 


IGNEOUS  ROCK  TYPES  —  VOLCANIC  ROCKS     161 

1.  Soda  rhyolite  from  Berkeley,  California. 

2.  Liparite  from  Telkebanya,  Hungary. 

3.  Rhyolite  from  Hot  Springs  Hills,  Pahute  Range,  Utah. 

4.  Pantellerite,  Kahania,  Island  of  Pantelleria,  Mediterranean. 

Discussion  of  Analyses: 

1.  Rhyolites  have  the  highest  silica  content  of  any 
volcanic  rock  and  generally  higher  than  granite. 

2.  They  have  low  iron,  magnesia  and  lime  contents, 
due  to  the  scarcity  of  dark  minerals.     The  lime  comes 
from  the  acid  plagioclase. 

3.  Potash  generally  predominates  over  soda. 

4.  In  pantellerite,  soda  predominates  over  potash,  due 
to  the  presence  of  anorthoclase. 

THE  TRACHYTE  FAMILY. 

Mineralogical  Composition. — Glassy  orthoclase  (sana- 
dine).  Biotite,  hornblende,  augite,  diopside.  Magnetite 
and  titanite  as  common  accessories.  Volcanic  equivalent 
of  the  syenites. 

Texture. — Groundmass  usually  crystalline  of  sana- 
dine,  containing  sanadine  or  orthoclase  phenocrysts  in 
which  Carlsbad  twinning  is  evident.  Flow  structure 
often  conspicuous. 

Relationship. — Trachytes  pass  into  phonolites  with 
increase  in  soda.  They  grade  into  syenites  with  the  devel- 
opment of  granitoid  texture.  They  may  be  confused  with 
andesites  unless  the  striated  feldspar  of  the  latter  is  dis- 
tinguishable. 

Character. — The  name  trachyte  is  derived  from  a 
Greek  work  trachus,  meaning  "rough,"  because  of  the 
rough  character  of  the  first  rocks  of  this  type  which  were 
studied.  They  are  not  common,  and  are  found  in  the  fol- 
lowing type  localities:  in  the  volcanic  districts  of  Italy 


162  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

and  the  Auvergne,  along  the  Rhine,  in  the  Azores,  in  the 
Black  Hills,  in  Ouster  County,  Colorado,  and  in  Montana. 

Analyses  of  Trachytes  (See  under  Phonolites). 
Discussion   of   Analyses. — Compared   with   rhyolites, 
the  trachytes  show : 

1.  Lower  silica,  due  to  decrease  in  quartz. 

2.  Higher  alumina,  magnesia,  lime  and  iron,  due  to 
increase  in  ferro-magnesian  minerals. 

3.  Higher  alkalies,  potash  usually  predominating. 

THE  PHONOLITE  FAMILY. 

Mineralogical  Composition. — Sanadine  and  nephelite 
or  leucite.  Aegirite.  Occasionally  members  of  the  soda- 
lite  group.  Garnet  as  an  accessory. 

Texture. — The  groundmass  is  crystalline,  sometimes 
porphyritic,  rarely  glassy. 

Relationship. — Phonolite  grades  into  trachyte  with 
decrease  in  soda.  The  two  types  are  closely  associated. 

Character. — Phonolite  is  a  translation  into  Greek  of 
a  German  word  Klingstein,  or  "cluck  stone,"  so  named 
because  certain  phonolites  with  a  pronounced  horizontal 
jointing  when  hit  give  forth  a  metallic  sound.  The  rock 
has  a  greasy  appearance,  due  to  the  presence  of  nephe- 
lite. Nephelite  if  identified  serves  at  once  to  distinguish 
phonolite  from  other  volcanic  rocks. 

Leucite  phonolites  are  rare.  Leucite  may  and  fre- 
quently does  occur  with  nephelite  in  the  typical  phonolite. 
Concentrically  arranged  inclusions  of  magnetite  specks 
occur  in  the  leucite. 

The  pyroxenes  are  more  common  than  in  any  other 
volcanic  rock.  They  are  usually  segirite-augite  or  segirite 
in  long-tufted,  ragged,  bright  green  prisms.  The  acces- 


IGNEOUS  ROCK  TYPES  —  VOLCANIC  ROCKS 


163 


sory  minerals  sodalite,  brown  garnet,  and  titanite  are  in 
themselves  characteristic. 

Phonolites  are  relatively  not  common.  They  occur  in 
dikes,  sheets  and  isolated  buttes  (Devil's  Tower)  in  the 
Black  Hills  and  in  the  Cripple  Creek  mining  districts  of 
Colorado,  where  they  are  associated  with  purple  fluorite 
and  calaverite  in  the  ore  bodies.  The  phonolite  magmas 
being  rich  in  alkalies  may  have  had  a  solvent  effect  upon 
the  gold,  thus  accounting  for  the  present  association. 

In  Germany,  phonolites  occur  in  great  masses  as  vol- 
canic necks  or  plugs  in  southern  Baden,  near  the  Swiss 
border.  Many  old  castles  have  been  erected  on  the 
summits. 

Kilimanjaro,  one  of  the  volcanoes  which  has  recently 
been  active,  is  said  to  have  given  forth  phonolite  lavas. 


Analyses  of  Trachytes  and  Phonolites: 


TRACHYTES. 


PHONOLITES. 


SiO, 
Al-O, 


FeO 
MgO 
CaO 
Na,0 

H=O 

Total 
Sp.   Gr. 


1 

2 

3 

1 

2 

3 

66.30 

64.70 

66.03 

58.20 

58.50 

61.08 

17.80 

16.50 

18.49 

21.60 

19.70 

18.71 

2.30 

.70 

2.18 

21.80 

31.40 

1.91 

.40 

2.70 

.22 

.... 

.... 

.63 

.30 

1.70 

.39 

1.30 

.30 

.08 

2.10 

3.20 

.96 

2.00 

1.50 

1.58 

5.60 

2.70 

5.23 

6.00 

10.00 

8.68 

3.50 

5.50 

5.86 

6.60 

4.70 

4.63 

.20 

1.60 

.85 

2.10 

1.00 

2.21 

.  98.50       99.30     100.20 
2.6          2.56      .  2.59 


97.56       99.10       99.51 
2.6  2.58 


TRACHYTES. 

1.  Trachyte  from  Auvergne,  France. 

2.  Biotite  hypersthene  trachyte  from  Tuscany,  Italy. 

3.  Trachyte  from  Game  Ridge,  Custer  County,  Colorado. 


164  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

PHONOLITES. 

1.  Phonolite  from  Schlpssberg,  Teplitz,  Bohemia. 

2.  Phonolite  from  Miaune,  France. 

3.  Phonolite  from  "Devil's  Tower,"  Black  Hills,  Wyoming. 

Discussion  of  Analyses. — Compared  with  trachytes, 
the  phonolites  show : 

1.  Lower  silica  due  to  the  substitution  of  nephelite 
for  sanadine. 

2.  Higher  alumina  and  alkalies. 

3.  Lower  iron,  magnesia  and  lime,  due  to  absence  of 
dark  minerals.    In  case  aegirite  is  present,  soda  and  iron 
are  increased. 

4.  Traces  of  chlorine  and  sulphur  are  due  to  the  pres- 
ence of  members  of  the  sodalite  group. 

THE  DACITE  AND  ANDESITE  FAMILY. 
Mineralogical  Composition. — Acid  plagioclase ;  biotite, 
hornblende,  augite,  diopside ;  magnetite,  apatite,  zircon  as 
common  accessories.     Quartz  is  present  in  dacite  and 
absent  in  andesite. 

Texture. — Groundmass  present  as  glass  or  as  an  inti- 
mate mixture  of  minute  indistinguishable  feldspars, 
which  may  be  described  as  a  "pepper  and  salt"  texture. 
Plagioclase  feldspar  shows  irregular  outlines  with  zonal 
arrangement  of  inclusions  frequent. 

Character. — This  group  is  the  volcanic  equivalent  of 
the  quartz  diorite  and  diorite  group.  The  dacites  are  not 
common.  They  were  named  from  an  old  Roman  province 
of  Dacia,  now  a  part  of  Hungary.  Andesites  derived 
their  name  from  the  abundance  of  lava  of  this  type  in 
the  Andes  Mountains. 

Differentiation  from  other  volcanic  rocks  may  be 
based  upon  the  peculiar  "pepper  and  salt"  texture.  Da- 


IGNEOUS  ROCK  TYPES  —  VOLCANIC  ROCKS     165 

cite  and  rhyolite  are  confused  unless  the  twinning  of  the 
plagioclase  of  the  former  is  observed. 

Among  some  of  the  active  volcanoes  which  furnish 
andesite  lavas  are :  Chimborazo,  in  the  Andes ;  Aphro- 
essa  in  the  Santorin  Archipelago,  Aegean  Sea,  which  was 
in  eruption  in  1863;  Krakatoa,  whose  last  eruption  was 
in  1883,  and  the  extinct  volcanoes  Mount  Shasta,  Mount 
Hood  and  Mount  Rainier. 

Classification  of  the  Andesites: 

1.  Mica  andesite. 

2.  Hornblende  andesite. 

3.  Pyroxene  andesite. 

a.  Augite  andesite. 

b.  Hypersthene  andesite. 

Analyses  of  Dacite  and  Andesite  (See  next  table). 

Discussion  of  Analyses. — Compared  with  trachytes 
the  analyses  show : 

1.  Lower  silica,  due  to  the  substitution  of  acid  plagio- 
clase for  alkali  feldspar. 

2.  Higher  alumina,  iron,  magnesia  and  lime,  due  to 
the  presence  of  the  dark  minerals. 

3.  Lower  alkalies.     Soda  always  predominates  over 
potash.    This    is    due    to    the    presence    of    the    acid 
plagioclase. 

THE  BASALT  FAMILY,  INCLUDING  DIABASE. 
Mineralogical    Composition. — Basic    plagioclase,    au- 
gite;  magnetite  is  a  common  accessory.    Olivine  is  pres- 
ent in  olivine  basalt. 

Texture. — These  rocks  possess  a  texture  that  is  char- 
acteristic of  rapid  cooling.  They  have  occasionally  a 
glassy  groundmass  dotted  with  skeleton  crystals,  but  more 
commonly  they  are  porphyritic.  The  more  crystalline 


166  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

portion  of  the  rock  consists  of  prominent  augite  and  oliv- 
ine  crystals  with  good  outline,  and  with  plagioclase  poorly 
developed  in  small  crystals.  The  pale  buff  color  of  the 
augite  phenocrysts  is  characteristic. 

The  texture  of  diabase  is  intermediate  between  that 
of  gabbro  and  of  basalt.  It  is  essentially  an  intrusive 
basalt,  entirely  crystalline  and  granitoid.  The  plagio- 
clase crystals  are  idiomorphic,  occurring  in  long,  lath- 
shaped  crystals  which  lie  in  all  positions.  The  interstices 
are  filled  with  allotriomorphic  crystals  of  augite  and  mag- 
netite. This  texture  is  called  "ophitic."  It  is  an  impor- 
tant microscopic  criterion  for  the  identification  of 
diabase. 

Diabase  is  variously  classified,  sometimes  as  a  plutonic 
rock  and  sometimes  as  a  volcanic  rock.  Since  it  grades 
into  porphyritic  forms  at  the  contacts  and  since  it  is 
really  volcanic  in  its  nature,  occurring  in  sheets  or 
dikes  of  limited  thickness  close  to  the  surface,  it  is 
considered  here  with  the  basalts. 

Basalts  are  volcanic  equivalents  of  the  gabbros.  They 
are  difficult  to  classify  in  the  field,  as  they  are  all  heavy, 
black  gray  or  brown  rocks  for  which  a  common  and  use- 
ful field  term  "dolerite"  has  been  applied.  The  term  dia- 
base originated  from  the  Greek  verb  diabaitiein,  "to  pen- 
etrate." Trap  is  a  common  field  synonym  applied  to 
rocks  of  diabasic  texture. 

Classification. — A  simple  classification  of  the  basalt 
family  which  meets  all  field  requirements  is : 

1.  Basalt. 

2.  Olivine-basalt. 

3.  Diabase. 

4.  Olivine-diabase. 

Distribution. — Basalts  are  abundant  particularly  along 
the  Atlantic  seacoast  where  diabase  has  intruded  Triassic 


IGNEOUS  ROCK  TYPES  —  VOLCANIC  ROCKS 


167 


shales.  It  has  formed  prominent  landmarks,  such  as  the 
Palisades  of  the  Hudson,  East  and  West  Rock  near  New 
Haven,  and  Deep  River,  North  Carolina.  Thousands  of 
feet  of  basalt  of  Pre-Cambrian  age  are  found  on  Kewee- 
naw  Point.  Native  copper,  secondarily  precipitated  in 
the  amygdaloidal  cavities  of  these  flows,  is  an  important 
ore.  The  Columbia  Plateau  and  the  Deccan  Plateau  fur- 
nish the  two  greatest  examples  of  basaltic  extrusion. 

The  lavas  from  many  volcanoes  are  chiefly  basaltic. 
Among  these  are  Kilauea  and  Mauna  Loa  in  the  Hawaiian 
Isands,  Mount  Etna,  and  various  volcanoes  in  Iceland. 


Analyses  of  Dacite,  Andesite,  Basalt  and  Diabase: 


SiO, 
A1.CX 


FeO 

MgO 

CaO 

Na20 

K20 

H20 

Total 


1 

2 

3 

4 

5 

6 

69.40 

62.00 

60.30 

51.80 

49.70 

49.20 

16.20 

17.80 

16.90 

12.80 

13.60 

13.50 

.90 



5.90 

3.60 

7.80 

5.50 

1.50 

4.40 

1.40 

8.70 

7.20 

10.60 

1.30 

2.60 

3.50 

7.60 

5.50 

6.80 

3.20 

5.40 

5.60 

10.70 

12.40 

11.50 

4.10 

4.30 

3.80 

2.10 

1.60 

1.80 

3.00 

1.50 

2.40 

.40 

1.20 

.10 

.40 


1.70 


.40 


.60 


.10 


.30 


100.00       100.99       100.20         98.30 


99.10         99.80 


1.  Dacite  from  Lassen's  Peak,  California. 

2.  Hypersthene  andesite  from  Mount  Shasta,  California. 

3.  Augite  andesite  from  Chimborazo,  Mexico. 

4.  Diabase  from  New  Haven,  Connecticut. 

5.  Basalt  lava  from  Thjorsa,  Iceland. 

6.  Iron  basalt  from  Nifak,  Disco  Island,  Greenland. 

Discussion  of  Basalt  Analyses. — When  compared  with 
andesites,  the  analyses  show: 

1.  Lower  silica. 

2.  Higher  alumina. 


168  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

3.  Lower  alkalies,  all  due  to  the  increasing  basicity 
of  the  feldspar. 

4.  Higher  iron,  magnesia  and  lime,  due  to  the  addi- 
tion of  dark  minerals. 

5.  Soda  predominates  over  potash. 

TRACHYDOLERITES. 

Mineralogical  Composition. — Basic  plagioclase  and 
alkali  feldspar ;  pyroxene ;  members  of  the  sodalite  group, 
olivine,  and  hornblende. 

Texture. — Often  porphyritic,  with  phenocrysts  of 
basic  plagioclase. 

Relationship. — Trachydolerites  are  the  volcanic  equiv- 
alents of  essexites.  They  are  intermediate  between  alkali 
trachytes  and  phonolites  on  one  side  and  tephrites  on  the 
other. 

Analyses  (See  next  table). — The  analyses  show  low 
silica,  high  alumina,  iron,  magnesia,  lime,  and  the 
alkalies. 

TEPHRITES  AND  BASANITES. 

Mineralogical  Composition. — Basic  plagioclase  and 
either  nephelite  or  leucite.  Augite  is  common.  Basanite 
contains  olivine  and  tephrite  does  not. 

Relationship. — These  rocks  are  the  volcanic  equiva- 
lents of  the  theralites  and  the  shonkinites. 

Classification. — Basic  plagioclase  is  common  to  all 
varieties. 

1.  Leucite  tephrite,  with  addition  of  leucite  and  au- 
gite. 

2.  Leucite  basanite,  with  addition  of  leucite,  augite 
and  olivine. 

3.  Nephelite  tephrite,  with  addition  of  nephelite  and 
augite. 


IGNEOUS  ROCK  TYPES  —  VOLCANIC  ROCKS     169 

4.  Nephelite  basanite,  with  addition  of  nephelite, 
augite  and  olivine. 

The  lavas  of  Mount  Vesuvius  are  leucite  tephrites, 
containing  a  little  olivine,  thus  grading  toward  the  leu- 
cite  basanites. 

Analyses  (See  next  table). — The  chemical  character  of 
this  group  is  very  similar  to  that  of  the  last  group.  The 
alkalies  are  higher,  due  to  the  addition  of  leucite  and 
nephelite. 

LEUCITITES  AND  LEUCITE  BASALTS. 
NEPHELITITES  AND  NEPHELITE  BASALTS. 

Mineralogical  Composition. — Leucite  or  nephelite 
with  augite;  with  or  without  olivine.  Nonfeldspathic. 
They  grade  from  tephrites  and  basanites  by  the  elimina- 
tion of  the  basic  feldspar. 

Relationship. — These  rocks  are  the  volcanic  equiva- 
lents of  the  ijolites  and  the  missourites.  The  nephelite 
basalts  accompany  the  phonolites  in  the  Cripple  Creek 
district  of  Colorado,  and  are  sometimes  completely 
impregnated  with  gold  telluride. 

Analyses  (See  next  table). — The  chemical  character 
of  this  group  corresponds  closely  to  that  of  the  tephrite 
and  basanite  group  if  the  characteristics  normally  con- 
tributed by  the  basic  plagioclase  are  deducted. 

LIMBURGITE  AND  AUGITITE. 

Mineralogical  Composition. — Limburgite  consists  of 
pyroxene  and  olivine,  and  augitite  consists  of  pyroxene 
only.  They  both  contain  apatite  and  magnetite  as  com- 
mon accessories.  They  contain  no  feldspars  nor  feld- 
spathoids. 

Relationship. — These  rocks  are  regarded  as  the  most 
basic  of  the  volcanic  rocks  and  are  the  end  products  of 
the  trachydolerite-tephrite-leucitite  series.  They  may  be 


170  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

regarded  as  the  volcanic  equivalents  of  peridotite  and 
hornblendite. 

Analyses. — Chemically  they  show  a  decrease  in  the 
alkalies,  due  to  the  absence  of  the  feldspathoids.  Lim- 
burgite  shows  an  increase  in  magnesia,  due  to  the  pres- 
ence of  olivine. 


SiO, 

A12O; 

Fe20: 

FeO 

MgO 

CaO 

Na.O 

K20 

H20 


1 

2 

3 

4 

5 

6 

7 

51.76 

48.09 

47.40 

45.90 

46.90 

42.78 

43.35 

16.64 

20.12 

23.70 

18.70 

21.60 

8.66 

11.46 

14.06 

6.72 

6.80 

.... 

8.10 

.... 

11.98 

4.32 

3.50 

10.70 

.... 

17.96 

2.26 

3.21 

4.19 

1.90 

5.70 

2.50 

10.06 

11.69 

8.15 

9.37 

6.50 

10.60 

8.00 

12.29 

7.76 

4.98 

2.62 

6.40 

1.70 

8.90 

2.31 

2.88 

1.31 

5.69 

3.30 

6.80 

2.60 

.62 

.99 

1.70 

.60 

2.10 

3.96 

2.41 

Total      .      100.11     101.12     101.20     100.60     100.10       98.64       95.80 

1.  Trachydolerite  from  Chajorra,  Island  of  Teneriffe.     Erup- 

tion of  1798. 

2.  Leucite  tephrite  from  Atreo  del  Cavallo,  Vesuvius.     Erup- 

tion of  May,  1855. 

3.  Nephelite  tephrite  from  lava  stream  on  San  Antao,  Cape 

Verde  Islands. 

4.  Leucitite  from  Capo  de  Bova,  Via  Appia,  Rome. 

5.  Nephelinite  from  San  Antao,  Cape  Verde  Islands. 

6.  Limburgite  from  Limburg,  Kaiserstuhl  Mountains,  Baden, 

Germany. 

7.  Augitite  from  Hutberg,  near  Tetscfcen,  Germany. 

PYROCLASTIC  ROCKS. 

Pyroclastic  rocks  are  those  made  up  of  fragmental 
volcanic  deposits,  usually  more  or  less  stratified  by  trans- 
portation through  the  air  or  under  water. 

Classification. — 1.  Volcanic  agglomerate  consists  of 
the  large  sized  volcanic  products  of  a  fragmental  nature 
which  have  been  deposited  near  the  crater. 


IGNEOUS  ROCK  TYPES  —  VOLCANIC  ROCKS     171 

2.  Volcanic  breccia  consists  of  angular  fragmental 
products  which  have  been  firmly  cemented  together. 

3.  Tuff  is  a  deposit  of  volcanic  ash  or  dust  consoli- 
dated by  cementation.     In  the  historical  eruption   of 
Vesuvius,  in  79  A.D.,  Pompeii  was  buried  in  ash  and  Her- 
culaneum  was  buried  in  tuff.     Excavation  has  gone  on 
rapidly  in  Pompeii,  as  the  loose  ash  offers  less  resistance 
to  removal  than  does  the  tuff  which  covered  Herculaneum. 


172  OPTICAL  MINERALOGY  AND  PETROGRAPHY 


CHAPTER  11. 
SEDIMENTARY  AND  METAMORPHIG  ROCKS. 

Sedimentary  rocks  are  of  secondary  origin  in  that 
they  are  formed  from  previously  existing  rocks  which 
may  have  been  either  igneous,  sedimentary,  or  metamor- 
phic.  They  may  be  mechanically  or  chemically  deposited, 
either  on  land  or  under  water.  The  agents  of  deposi- 
tion are  water,  wind,  and  ice. 

By  the  weight  of  overlying  strata  and  through  the 
agency  of  siliceous,  calcareous  or  ferruginous  cement, 
they  become  consolidated  from  a  loose  aggregate  to  a 
solid  mass. 

CLASSIFICATION. — 1.  Sediments  of  mechanical  origin. 

A.  Water  deposits. 

a.  Conglomerate. 

b.  Breccia. 

c.  Sandstone. 

d.  Shales. 

B.  Wind  deposits. 

a.  Loess. 

b.  Sand  dunes. 

2.  Sediments  of  chemical  origin  formed  from 
solution. 

A.  Concentration, 
a.  Sulphates. 
Gypsum. 
Anhydrite. 


SEDIMENTARY  AND  METAMORPHIC  ROCKS  173 

b.  Chlorides. 
Halite. 

c.  Silica. 
Flint,  etc. 

d.  Carbonates. 
Limestone,  etc. 

e.  Oxides. 

Iron  Ores. 

B.  Organic,  through  the  agency  of  animals  and 
plants. 

a.  Carbonates. 

Limestones  of  several  kinds. 

b.  Silica. 
Diatomaceous  earth,  etc. 

c.  Phosphate. 

Phosphate  rock. 

d.  Carbon. 

Coal,  etc. 
Sedimentary  Rocks  of  Mechanical  Origin. 

Conglomerates. — Conglomerate  is  a  rock  consisting  of 
cemented  fragments  of  rounded,  water-worn  material  of 
varying  sizes.  The  pebbles  are  usually  made  of  the  more 
resistant  varieties  of  minerals  and  rocks.  Finer  sedi- 
ments fill  the  interstices.  Conglomerates  are  aqueous  in 
origin  and  show  more  or  less  stratification.  The  repre- 
sent near-shore  conditions  of  sedimentation. 

Breccias. — A  breccia  is  a  rock  composed  of  angular 
fragments  cemented  into  a  solid  mass. 

1.  Talus  breccia  is  derived  by  ordinary  weathering 
and  disintegration  of  a  rock  ridge.     It  accumulates  at 
the  base  of  slopes  or  cliffs. 

2.  Fault  or  friction  breccia  is  derived  from  earth's 
movements,  which  crush  the  rock  on  two  sides  of  a  fault 
plane  by  friction  or  by  intense  pressure. 


174  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

3.  Volcanic  breccia  is  formed  by  an  accumulation  of 
angular  fragments  ejected  by  volcanic  action  and  later 
solidified. 

Sandstone. — Sandstone  is  composed  of  sand  grains 
which  have  been  rounded  by  water  action  and  separated 
by  the  classifying  action  of  moving  water  to  deposits  of 
uniform  texture.  Quartz  is  the  essential  constituent, 
although  impurities  are  always  present,  such  as  feldspar, 
mica,  garnet,  magnetite,  etc. 

CLASSIFICATION. — According  to  the  character  of  the 
cement  : 

1.  Siliceous  sandstone. 

2.  Ferruginous  sandstone. 

3.  Calcareous  sandstone. 

4.  Argillaceous  sandstone. 

According  to  mineral  content : 

1.  Arkose,  containing  much  feldspar.- 

2.  Graywacke,   containing  ferro-magnesian  min- 
erals. 

3.  Micaceous  sandstone,  etc. 

Shale. — Shale  is  a  rock  consisting  of  the  finer  mate- 
rial, usually  clayey,  deposited  beyond  the  zone  of  depo- 
sition of  the  sandstone.  It  contains  compacted  clays, 
muds  and  silts,  which  possess  a  finely  stratified  structure 
called  bedding. 

CLASSIFICATION. — According  to  composition. 

1.  Argillaceous  shales. 

2.  Arenaceous  shales. 

3.  Ferruginous  shales. 

4.  Carbonaceous  shales. 

Shales  form  about  87  per  cent  of  the  sedimentary 
rocks,  sandstones  about  8  per  cent,  and  limestones  about 
5  per  cent. 


SEDIMENTARY  AND  METAMORPHIC  ROCKS  175 

Loess. — Loess  is  a  fine,  homogeneous,  clay-like  sub- 
stance, largely  siliceous,  which  lacks  all  semblance  of 
stratification,  and  when  eroded  forms  precipitous  cliffs. 
It  contains  angular  quartz,  mica  flakes,  clayey  material, 
with  often  high  percentages  of  calcium  carbonate. 

Loess  is  believed  to  be  a  wind-blown  deposit,  probably 
assisted  in  certain  localities  by  aqueous  deposition.  It  is 
used  in  the  West  for  brick  manufacture. 

Adobe  clay  is  a  form  of  loess  abundant  in  the  arid 
southwestern  portion  of  the  United  States.  It  is  used  in 
the  manufacture  of  sun-dried  brick  for  adobe  houses. 

Sand  Dunes. — Sand  dunes  are  formed  by  wind-blown 
sand,  which  always  exhibits  a  characteristic  shape  with 
its  long,  gentle  slope  on  the  windward  side,  up  which  the 
sand  grains  are  blown,  and  with  a  steeper  slope  on  the 
leeward  side,  which  is  the  angle  of  repose  for  sand  grains. 
Sand  dunes  show  stratification  and  ripple  marks. 

The  migration  of  sand  dunes  has  been  known  to  create 
havoc  in  certain  parts  of  the  coutry.  They  are  "fixed" 
by  transplanting  with  beach  grass  and  sand  hedges.  One 
of  the  railroads  temporarily  checked  the  progress  of  some 
advancing  sand  dunes  by  spraying  them  with  crude 
petroleum. 

Sediments  of  Chemical  Origin. 

Gypsum  and  Anhydrite. — Gypsum  and  anhydrite,  the 
hydrous  and  the  anhydrous  sulphates  of  lime,  occur  inter- 
bedded  or  in  irregular  masses,  interstratified  with  clays, 
shales,  sandstones,  and  limestones,  or  with  halite. 

They  originate  from  concentration  of  oceanic  waters 
by  evaporation,  and  in  inland  lakes  in  which  the  evapora- 
tion equals  or  exceeds  the  amount  of  inflow. 

Anhydrite  changes  to  gypsum  by  normal  hydration, 
due  to  exposure.  A  tunnel  in  Europe  which  was  driven 


176  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

through  anhydrite  was  thrown  out  of  alignment  by  the 
volume  increase  produced  by  this  change. 

Halite. — Halite  occurs  in  massive,  granular  form, 
interstratified  with  clays,  marl  and  sandstone.  It  is  espe- 
cially associated  with  gypsum,  anhydrite  and  dolomite. 
The  deposits  of  Strassford,  Germany,  are  4,000  feet  thick. 
It  is  here  associated  with  the  chlorides  and  sulphates  of 
potassium  and  magnesium. 

Flint. — Flint  is  a  cryptocrystalline  variety  of  silica 
occurring  as  a  hard,  grayish  to  blackish  rock,  its  color 
being  due  to  carbonaceous  matter.  It  occurs  as  nodules 
or  lenses  in  limestones.  It  is  used  for  road  material,  and 
in  tube  and  ball  mills. 

Sediments  of  Organic  Origin. 

Limestone. — Limestone  is  a  widely  distributed  cal- 
cium carbonate  rock  containing  impurities  of  magnesia, 
silica,  clay,  iron,  and  organic  matter.  It  is  quite  soluble, 
and  allows  the  formation  of  sink  holes,  caves,  solution 
cavities,  etc.  Buildings  erected  of  limestone  thirty  or 
forty  years  ago  often  show  the  effect  of  weathering  by 
pitted  and  etched  surfaces. 

Limestone  forms  by  chemical  precipitation  and 
through  the  agency  of  animals  and  plants. 

CLASSIFICATION. — The  classification  of  limestone  is 
based  upon  composition,  texture,  and  uses.  It  has  a  wide 
range  of  occurrence. 

1.  Calcic  limestone  is  chiefly  calcium  carbonate. 

2.  Dolomite  refers   usually  to   any   magnesian   rich 
limestone. 

3.  Chalk  is  a  soft,  porous,  fine-grained  variety  com- 
posed of  minute  shells  of  foraminifera. 

4.  Hydraulic  limestone  is  a  clayey  limestone  used  in 
cement  manufacture. 


SEDIMENTARY  AND  METAMORPHIC  ROCKS  177 

5.  Lithographic  limestone  is  a  fine-grained,   homo- 
geneous variety  used  for  lithographic  work. 

6.  Oolitic  limestone  is  composed  of  small  spherical 
grains  of  calcium  carbonate. 

7.  Travertine  is  a  porous,  cellular  variety  deposited 
by  hot  springs. 

Iron  Ores. — The  oxides  of  iron,  hematite  and  limonite, 
and  the  iron  carbonate,  siderite,  may  all  have  a  sedimen- 
tary origin,  either  secondary  or  primary.  They  are  all 
commercially  valuable  as  a  source  of  iron. 

Phosphate  Rock. — Phosphate  rock  consists  chiefly  of 
calcium  phosphate.  It  has  a  value  as  a  source  of  phos- 
phoric acid  in  the  manufacture  of  fertilizers.  It  is  of 
organic  origin,  formed  from  animal  remains.  Large 
deposits  are  found  in  Florida,  Tennessee,  Idaho,  Wyo- 
ming, and  Utah. 

Carbonaceous  Rocks. — These  rocks  include  all  accu- 
mulations of  vegetable  matter  that  have  undergone  par- 
tial or  complete  decay  under  water. 

The  principal  varieties  which  form  transitional  stages 
from  the  unaltered  plant  remains  to  graphite  by  steadily 
increasing  metamorphism  are  peat,  lignite  or  brown  coal, 
bituminous  or  soft  coal,  and  anthracite  or  hard  coal. 

Metamorphic  Rocks. 

Agents  of  Metamorphism. — The  chief  agents  involved 
in  the  alteration  of  igneous  and  sedimentary  rocks  to 
their  metamorphic  equivalents  are:  (1)  dynamic  action 
due  to  earth  movements  producing  shearing  and  folding 
of  the  rock  formations,  and  (2)  chemical  action  influ- 
enced by  heat,  liquids,  and  gases. 

Composition  of  Metamorphic  Rocks. — The  chemical 
composition  of  metamorphic  rocks  is  frequently  similar 


178  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

to  the  composition  of  the  rocks  from  which  they  are 
derived.  In  so  far  as  this  is  true,  chemical  analysis  is 
an  important  criterion  for  discriminating  between  meta- 
morphosed sedimentary  and  metamorphosed  igneous 
rocks. 

Frequently  there  is  addition  or  subtraction  of  con- 
stituents accompanying  metamorphism  which  renders 
more  difficult  the  interpretation  of  the  origin  of  the  meta- 
morphic  rock. 

Mineral  composition  may  or  may  not  be  the  same  in 
the  metamorphic  rock  as  it  was  in  the  original  rock.  Fre- 
quently metamorphism  is  accomplished  by  a  granulation 
and  rotation  of  the  original  particles.  In  the  greater 
number  of  cases,  there  is  a  development  of  platy  minerals 
which  are  best  adapted  to  withstand  conditions  of  higher 
pressures  and  temperatures.  In  these  minerals  the  mutual 
parallelism  of  the  greatest,  mean  and  least  dimensional 
axes  causes  a  more  or  less  perfect  cleavage  in  one  plane, 
which  is  called  schistocity.  The  average  ratio  of  the 
greatest  to  the  mean  dimensions  of  mica  is  10  :  1,  of 
hornblende  4  :  1,  and  of  quartz  and  feldspar  1.5  :  1. 

A  metamorphic  rock  contains  a  higher  percentage  of 
.the  minerals  mica  and  hornblende  than  the  original  rock. 
For  example,  shale  may  contain  no  mica.  By  metamor- 
phism, mica  schist  is  developed,  containing  over  50  per 
cent  of  mica.  No  change  in  chemical  composition  has 
taken  place.  Obviously  the  mica  was  developed  by  a 
recrystallization  of  the  constituents  originally  contained 
in  the  rockmass. 

Minerals  which  are  characteristic  of  metamorph:c 
rocks  are  staurolite,  cyanite,  sillimanite,  zoisite,  chlorite, 
talc,  etc.  Quartz,  feldspar,  mica,  pyroxene,  and  amphi- 
bole  are  common  to  both  igneous  and  metamorphic  rocks. 


SEDIMENTARY  AND  METAMORPHIC  ROCKS  179 

Criteria  for  the  Discrimination  of  Metamorphosed 
Igneous  and  Metamorphosed  Sedimentary  Rocks: 

1.  Mineralogical  composition. 

The  minerals  which  are  strongly  indicative  of  a  sedi- 
mentary origin  of  the  metamorphic  rocks  in  which  they 
occur  are  staurolite,  andalusite,  sillimanite,  cyanite.  They 
all  contain  higher  percentages  of  alumina  than  those 
found  in  igneous  rocks,  and  as  alumina  is  almost  insoluble 
there  is  practically  no  possibility  of  an  addition  of  alu- 
mina from  other  sources. 

2.  Original  textures  and  structures. 

If  not  too  severely  metamorphosed,  sedimentary  rocks 
may  show  remnants  of  bedding,  fossils,  cross-bedding,  or 
other  features.  Igneous  rocks  may  show  amygdaloidal 
cavities,  flow  structure,  etc. 

3.  Field  relationships. 

Areal  distribution  and  association  of  the  metamor- 
phosed rock  with  surrounding  rocks  may  give  some  clue. 
By  tracing  the  metamorphosed  rock  laterally  along  the 
strike,  one  may  come  to  a  less  metamorphosed  portion  of 
the  rock  which  still  shows  original  sedimentary  textures 
or  structures. 

4.  Chemical  composition. 

a.  Dominance  of  magnesia  over  lime  is  indicative  of 
sedimentary  origin. 

b.  Dominance  of  potash  over  soda  is  suggestive  of 
sedimentary  origin. 

c.  The  presence  of  several  per  cent  of  alumina  over 
the  1  :  1  ratio  necessary  to  satisfy  the  lime  and  alkalies 
is  suggestive  of  sedimentary  origin. 

d.  A  high  silica  content  is  suggestive  of  sedimentary 
origin  if  supported  by  other  criteria. 


180  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

CLASSIFICATION. — The  classification  of  metamorphic 
rocks  is  based  upon  composition,  texture,  and  structure : 

1.  Gneisses. 

2.  Schists. 

3.  Quartzites. 

4.  Slates  and  phyllites. 

5.  Marbles. 

6.  Ophicalcite,  serpentine,  and  soapstone. 

TABLE  OF  SEDIMENTARY  ROCKS  AND  THEIR 
METAMORPHIC  EQUIVALENTS. 


Loose   Sediments 
Gravel. 
Sand. 
Silt  and  clay. 
Lime  deposits. 

Consolidated 
Rock 
Conglomerate. 
Sandstone. 
Shale. 
Limestone. 

Metamorphic 
Rock 
Gneiss,  schist. 
Quartzite. 
Slate,  phyllite. 
Marble. 

TABLE  OF  IGNEOUS  ROCKS  AND  THEIR 
METAMORPHIC  EQUIVALENTS. 

Igneous  Rocks. 

Coarse-grained  feldspathic          Metamorphic  Rocks. 
rocks,  as  granite,  syenite, 
etc.  Gneiss. 

F  i  n  e-grained    feldspathic 

rocks,  as  tuff,  etc.  Gneiss'  schlst 

Basic    igneous    rocks,    as     0  ,  . 

diorite,  basalt. 

Gneiss.- -A  gneiss  is  a  banded  metamorphic  rock, 
either  of  igneous  or  sedimentary  origin,  in  which  the 
bands  are  mineralogically  unlike,  consisting  chiefly  of 
quartz  and  feldspar,  with  or  without  the  parallel  dimen- 
sional arrangement  necessary  for  rock  cleavage. 

Gneisses  are  developed  by  a  granulation,  rotation  and 
recrystallization  of  the  original  minerals  rather  than  by 


SEDIMENTARY  AND  METAMORPHIC  ROCKS  181 

the  development  of  an  entirely  new  set  of  the  platy,  cleav- 
able  minerals. 

CLASSIFICATION. — According  to  the  prevailing  acces- 
sory mineral: 

Biotite  gneiss. 

Muscovite  gneiss. 

Hornblende  gneiss,  etc. 
According  to  origin : 

Granite  gneiss. 

Gabbro  gneiss. 

Diorite  gneiss,  etc. 

Schist. — A  schist  is  a  foliated,  metamorphic  rock 
whose  individual  folia  are  mineralogically  alike,  and 
whose  principal  minerals  are  the  flat,  platy  minerals 
which  are  best  adapted  to  withstand  conditions  of  high 
pressure  and  high  temperature.  The  parallel  arrange- 
ment of  the  minerals  develops  the  capacity  to  part  along 
parallel  planes,  called  schistosity. 

CLASSIFICATION. — According  to  the  prevailing  schis- 
tose mineral : 

Chlorite  schist. 

Mica  schist. 

Talc  schist. 

Actinolite  schist,  etc. 

Quartzite. — Quartzite  is  developed  from  sandstone  by 
a  recrystallization  of  the  original  constituents  into  a  hard, 
compact,  crystalline  mass  having  a  splintery  or  con- 
choidal  fracture. 

A  pure  quartzite  is  rare,  although  the  percentages  of 
alumina,  iron  and  the  bases  are  often  small.  With  an 
increase  in  impurities,  the  quartzite  tends  to  take  on  a 
schistocity,  due  to  the  formation  of  the  impure  constitu- 
ents by  recrystallization  into  the  flat,  platy  minerals. 
Quartzite  schist  is  such  a  transition  in  which  mica  scales 


182  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

are  found  along  the  foliation  planes.  These  planes  prob- 
ably represent  original  bedding  planes  in  the  sandstone. 

Quartzite  is  used  to  advantage  as  a  building  stone, 
although  its  extreme  hardness  is  found  to  be  a  handicap 
both  in  quarrying  and  in  dressing. 

Slate  and  Phyllite. — Slate  is  a  dense,  thinly  cleavable, 
homogeneous  rock,  whose  cleavage  pieces  are  mineral- 
ogically  unlike,  and  whose  mineral  grains  are  so  small  in 
size  as  not  to  be  distinguished  by  the  eye.  This  cleavage 
is  not  to  be  confused  with  original  bedding  planes.  It  is 
a  secondary  structure  produced  in  the  development  of 
the  secondary  minerals. 

Slates  are  composed  of  the  finest  particles  of  mineral 
matter  which  are  carried  in  suspension  and  deposited 
considerable  distances  from  shore.  Volcanic  ash  and  tuff 
more  rarely  give  rise  to  slate  deposits. 

Phyllite  is  the  next  step  in  the  metamorphism  of  a 
slate,  intermediate  between  slate  and  mica  or  sericite 
schist.  Quartz  and  mica  are  the  essential  minerals. 

Marble. — Marble  is  the  metamorphic  equivalent  of  a 
limestone  or  a  dolomite.  It  is  completely  recrystallized, 
and  when  pure  shows  the  development  of  large  rhombic 
calcite  crystals  or  fine  sparkling  surfaces. 

Few  original  limestones  are  pure.  The  metamor- 
phism of  an  impure  limestone  containing  silica,  clayey 
material,  iron  oxides  and  carbonaceous  matter  is  charac- 
terized not  only  by  the  recrystallization  of  calcium  car- 
bonate but  by  the  development  of  various  secondary  sili- 
cates, particularly  biotite,  wollastonite,  diopside,  tremo- 
lite,  actinolite,  grossularite,  and  hornblende.  At  least 
seventy  secondary  minerals  have  been  found  to  exist  in 
metamorphosed  limestones. 

When  pure,  marble  is  massive,  and  shows  no  indica- 


SEDIMENTARY  AND   METAMORPHIC  ROCKS  183 

tion  of  a  schistose  structure.  All  traces  of  fossils  and 
original  structures  are  obliterated. 

Ophicalcite. — Ophicalcite  is  a  variety  of  marble  asso- 
ciated with  streaks  and  spots  of  serpentine.  Verde 
antique  is  a  name  more  popularly  used.  It  results  from 
the  metamorphism  of  an  originally  impure  limestone  to  a 
calcite-silicate  rock  in  which  the  silicates  were  later 
altered  by  hydration  to  serpentine.  Ophicalcite  is  valu- 
able for  decorative  purposes,  as  it  takes  an  easy  polish. 
It  occurs  in  quantities  in  Quebec,  in  the  Green  Mountains, 
and  in  the  Adirondacks. 

Serpentine. — Serpentine  rock  consists  essentially  of 
the  mineral  serpentine,  a  hydrous  magnesium  silicate,  in 
association  with  olivine,  pyroxene,  hornblende,  magnetite, 
chromite  and  the  carbonates.  Garnets  and  micas  are 
common  accessories. 

Serpentine  is  derived  by  metamorphism  of  igneous 
or  other  metamorphic  rocks  which  are  essentially  com- 
posed of  magnesium  silicates,  as  olivine,  pyroxene,  or 
hornblende.  Such  rocks  are  basic  igneous  rock  and  horn- 
blende schist. 

Serpentine  occurs  in  the  crystalline  area  of  eastern 
United  States,  in  eastern  Canada,  and  in  a  few  of  the 
western  coast  States,  but  seldom  in  large  masses.  It  is 
used  as  an  ornamental  stone  and  as  a  source  of  asbestos. 

Soapstone. — Soapstone  is  essentially  the  mineral  talc. 
It  becomes  a  talc  schist  by  taking  on  a  foliated  structure. 
Impurities  are  mica,  chlorite,  tremolite,  enstatite,  mag- 
netite, quartz,  and  pyrite. 

Soapstone  has  a  similar  origin  to  serpentine  as  a  sec- 
ondary product  from  the  magnesium  silicates.  It  is  found 
in  association  with  talcose  and  chloritic  rocks  in  crystal- 
line areas. 


184  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

Soapstone  is  mined  extensively  in  Virginia.  The  rock 
has  many  uses.  It  goes  into  the  manufacture  of  tubs, 
switchboards,  insulators,  sinks,  stoves,  fire-brick  and 
lubricants. 


SUGGESTIONS  FOR  GEOLOGICAL  WORK       185 


APPENDIX. 

Suggestions  for  Geological  Work. 

The  necessity  for  constant  and  careful  observation 
cannot  be  too  insistently  urged  in  an  examination  of  rocks 
or  geological  structures,  whether  it  be  a  prelimiary  recon- 
noissance  or  a  final  detailed  survey  of  a  property  of  lim- 
ited extent.  The  geologist  or  mining  engineer  who  is 
doing  geological  mapping  should  adopt  the  attitude  that 
it  may  be  impossible  to  return  to  the  particular  outcrop 
upon  which  he  is  working.  Every  feature  of  the  rock 
which  may  be  of  possible  value  in  the  solution  of  the 
problem  involved  should  be  recorded  on  the  spot.  This 
outcrop  may  prove  to  be  the  keystone  for  the  interpreta- 
tion of  the  structure  of  the  entire  area.  A  close  applica- 
tion of  this  rule  will  save  much  useful  time  and  energy. 

Observations  for  Geological  Mapping. 
The  following  outline  (from  Farrell)  may  be  used  as 
a  guide  to  the  geological  features  to  be  observed  in  an 
examination  of  a  'property : 
A.  RECONNOISSANCE  OF  THE  AREA. 

1.  Is  the  geology  simple  or  complicated? 

a.  Do  the  different  formations  cover  a  large  or 

a  small  area? 

b.  Is  it  easy  to  distinguish  between  them? 

c.  Are  the  boundaries  easy  to  find  and  follow? 

2.  What  are  the  probable  rock  types  and  their  rela- 
tions ? 

a.  Are  rocks  of  igneous  or  sedimentary  origin  or 
both? 


186  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

b.  Are  contacts  conformable  or  unconf ormable  ? 

c.  In  case  of  intrusive  bodies,  are  there  large  dikes 

or  small  masses  and  few  in  number,  or  are  they 
small  and  widely  distributed  through  the 
intruded  formations? 

d.  Are  the  rocks  much  altered? 

e.  Is  metamorphism  a  prominent  feature? 

3.  Collect  specimens  of  the  different  formations,  giv- 
ing locations  as  closely  as  possible. 

4.  Note  roads,  trails,  water,  and  possible  camping 
places. 

B.  GEOLOGICAL  MAPPING — GENERAL. 

1.  Locate  boundaries  between  formations. 

a.  Simple  boundaries. 
Take  dip  and  strike. 

Does  boundary  indicate  conformable  or  uncon- 

f ormable  contact? 
Are  there  evidences  of  faulting? 

b.  Obscure  boundaries. 

Look  for  fragmental  traces  of  the  formations. 
Work  up  hill  and  locate  the  highest  points  at 

which  fragments  of  the  lower  formation 

appear. 
Note  whether  scarps  or  change  of  slope  are 

connected  with  the  boundary. 

c.  Complicated  boundaries. 

Intrusive  boundaries. 

Map  carefully  dikes  and  arms. 

Note  alteration  and  metamorphism  in  the 

neighborhood  of  the  boundary. 
Note  variations  in  texture  of  the  igneous 

rock  in  approaching  the  boundary. 


SUGGESTIONS  FOR  GEOLOGICAL  WORK       187 

Boundaries  showing  contact  metamorphism. 
Map  the  general  relations  of  the  metamor- 

phic  patches. 
Note  the  metamorphic  minerals  and  their 

succession. 

Note  presence  and  association  of  ore  min- 
erals. 

2.  Work  within  the  boundaries  of  a  formation.    Trav- 
ersing. 

a.  In  areas  of  sedimentary  rocks. 

Strike  and  dip  of  beds. 

Color,  thickness  and  general  character  of  beds. 

Minerals  composing  the  rocks;  nature  of  the 
grains  or  fragments  (angular  or  rounded)  ; 
cementing  material.  In  conglomerates  look 
for  recognizable  fragments  of  earlier 
formations. 

Presence  of  fossils. 

Areas  of  alteration. 

Areas  of  metamorphism. 

Systems  of  folds — minor  folding  direction  and 
pitches  of  axes  of  folds — relations  of  fold- 
ing to  faulting. 

b.  In  areas  of  igneous  rocks. 

Rock  texture  and  variations  in  texture. 

Variations  in  composition. 

Segregations. 

Inclusions  of  other  rocks. 

Dip    and    strike    of   schistosity    or    gneissoid 

structure. 
Flow  structure. 

c.  In  areas  of  metamorphic  rocks. 

Is  rock  of  sedimentary  origin? 
Does  it  show  traces  of  bedding? 


188  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

Are  gneissoid  laminae  continuous,  sugges- 
tive of  sheared  beds  ? 

Are  the  grains  rounded  or  angular  in  out- 
line? What  are  their  relative  sizes? 

Are  minerals  such  as  to  suggest  erosion 

or  metamorphic  processes? 
Is  the  rock  of  igneous  origin? 

Are  the  minerals  typical  of  igneous  rocks  ? 

Are  the  gneissoid  laminae  noncontinuous, 

suggestive  of  sheared  minerals? 
Are  the  changes  suggestive  of  dynamic  action, 

chemical  action  or  both  ? 

Is  the  rock  texture  suggestive  of  folding 
and  shearing? 

Does  it  suggest  an  impregnation  and  meta- 
morphism  by  replacement  process,  due 
to  action  of  solutions  ? 

Is  the  rock  widely  different  in  structure  and 
composition  from  the  original  type? 

C.  GEOLOGICAL  MAPPING — DETAILED  WORK. 

1.  Faults. 

a.  Strike  and  dip. 

b.  Evidences    of    movement — slickensides,    striae 

(their  direction  and  dip),  gouge,  drag,  etc. 

c.  Cross  fracturing. 

2.  Veins  and  other  ore  bodies. 

a.  Strike  and  dip. 

b.  General  character  of  mineralization. 
Strong  or  weak. 

Oxidized  or  unoxidized  vein  material. 

c.  Minerals  and  groups  of  minerals. 

d.  Relative  age  of  minerals. 


SUGGESTIONS  FOR  GEOLOGICAL  WORK  189 

e.  Represent  exact  outline  of  ore  body  as  far  as 

possible. 

f.  Note  occurrence  of  branches  or  false  walls. 

g.  Note   character   and   extent   of  alteration   of 
country  rock. 

h.  Nature  and  extent  of  replacement  of  the  wall 
rock  by  the  ore. 

Criteria  of  Relative  Age. 

1.  Older  rocks  are  more  likely  to  have  been  deformed 
and  metamorphosed,  and  therefore  are  harder  to 
recognize. 

2.  Older  formations  are  normally  at  the  base  of  the 
series  of  sediments  and  flows. 

3.  Older  formations  are  represented  by  fragments  in 
later  formations. 

4.  Younger    formations    fill    erosion    irregularities, 

fractures,  fault  planes  and  caves  in  older  forma- 
tions. 

5.  Younger  formations  cut  the  older  ones  as  dikes, 
and  include  fragments  of  them. 

Table  for  the  Examination  of  Rocks  in  the  Laboratory. 

A.  IGNEOUS  ROCKS. 

1.  Texture,  historical  deductions,  etc. 

2.  Mineralogical  composition. 

a.  Accessory  minerals. 

b.  Essential  minerals. 

3.  Relative  age  of  minerals. 

a.  Minerals  formed  with  crystal  boundaries  are 
older  than  the  surrounding  ones  without  crys- 
tal boundaries. 


190  OPTICAL  MINERALOGY  AND  PETROGRAPHY 

b.  Included  minerals  are  older  than  the  ones  which 
include  them. 

c.  Minerals  abutting  without  crystal  boundaries 

are  of  the  same  age  approximately. 

d.  Intergrown  minerals  are  of  the  same  age. 

e.  Minerals  cutting  others  are  younger  than  those 

they  cut. 
4.  Alteration  and  metamorphism. 

a.  Degree  of  change,  extent  to  which  original  min- 

erals are  changed. 

b.  Character  of  the  change,  secondary  minerals 
due  to  alteration,  metamorphic  minerals. 

B.  SEDIMENTARY  ROCKS. 

1.  Relative  sizes  and  shapes  of  the  component  par- 
ticles. 

a.  Unassorted  material,  large  and  small  fragments 
together,  imply  that  the  source  of  the  material 
is  near  at  hand  or  that  the  transporting  agent 
is  very  powerful. 

b.  Angular  grains,  fresh  in  appearance,  indicate 

disintegration     without     decomposition,     and 
little  movement  from  the  source. 

c.  Rounded   grains,    fresh,    imply    disintegration 

and  transportation  of  the  material. 

d.  Sorted  material,  where  the  grains  are  similar 
in  mineralogical  character,  indicates  that  the 
deposits  were  made  some  distance  from  the 
source,  or  the  original  rock  disintegrated  and 
weathered  also,  differentiating  the  more  resist- 
ing minerals. 


SUGGESTIONS  FOR  GEOLOGICAL  WORK  191 

2.  Look  for  fragments  which  give  some  clue  as  to  the 

source  from  which  the  sedimentary  material  is 
derived. 

3.  Determine  the  character  and  probable  origin  of 
secondary  cementing  material. 

C.  WITH  METAMORPHIC  ROCK  TRY  TO  DETERMINE  : 

1.  The  nature  of  the  original  constituents  and  the 

original  rock.     Look  for  traces  of  original  min- 
erals in  form  or  cleavage. 

2.  The  nature  of  the  alteration. 

a.  Dynamic — folding,  shearing,  etc.,  distortion  of 

crystals  or  fragments. 

b.  Chemical  change — older  minerals  partially  dis- 

solved by  later  ones. 


INDEX. 


Absorption   66,  67 

Accessory  minerals   ...124,  129 

Acmite    99 

Actinolite    103 

Acute  bisectrix    60 

Adjustment  screws    .......     30 

Aegirite    99 

Albite 121 

Albite  twinning 114 

Alkali  granite    139 

Alkali-lime  granite 139 

Alkali-lime  syenite   145 

Alkali  syenite   146 

Allotriomorphic    128 

Amphibole 100 

Analyses 

Andesite 167 

Augitite    170 

Basalt    167 

Dacite    167 

Diabase    167 

Diorite  150 

Essexite    154 

Gabbro 152 

Granite    144 

Ijolite 154 

Leucite  syenite   148 

Leucitite    170 

Limburgite    170 

Malignite 154 

Missourite    154 

Nephelinite   170 

Nephelite  syenite 148 

Norite    .  .   152 


Peridotite    156 

Phonolite    163 

Pyroxenite    156 

Rhyolite    160 

Shonkinite    154 

Syenite   146 

Tephrite   170 

Theralite   154 

Trachydolerite    170 

Trachyte    163 

Analyzer   26 

Andalusite    86 

Andesite    164 

Angle  of  incidence 15 

Anhydrite    175 

Anisotropic  media.  .14,  41,  75,  86 

Anorthite    122 

Anorthosite     151 

Apatite    82 

Appendix 185 

Arkose   174 

Assimilation   131 

Augite    98 

Augitite 169 

Axes  of  ether  vibration . . . 

42,  49,  64,  66 

Basalt    165 

Basanite    168 

Bastite 91 

Baveno    114 

Becke  method 38 

Bertrand  lens 30,  51 

Biaxial  crystals.  14,  48,  52,  64,  86 


194 


OPTICAL  MINERALOGY  AND  PETROGRAPHY 


Biotite   107 

Birefringence  . .  40,  44,  48,  50,  60 

Bisectrix    49 

Breccia    173 

Calcite 44,  80 

Canada  balsam  .36,  42 

Carlsbad    114 

Cementing  material 174 

Centering  screws 27 

Chalk   176 

Chlorite  108 

Cleavage    34 

Clinochlore    110 

Coal    177 

Compensation  point   47 

Color 42,  44,45,  143 

Color  scale 45 

Concentration     151 

Conglomerate  173 

Contact  metamorphism 141 

Convergent  lens    27 

Corundum  78 

Critical   angle    16 

Cross  hairs 27,  30 

Cryptocrystalline    128 

Crystal  form   34 

Crystallization 128,  131 

Dacite   135,  164 

Diabase 135,  165 

Diallage    97 

Differentiation   131 

Diopside 96 

Diorite 149 

Dispersion    56 

Dolerite   166 

Dolomite 81,   176 

Double  refraction.  .18,  21,  40,  48 
Due  de  Chaulnes,  method  of  37 
Dunite  .155 


Enstatite 89 

Epidote    110 

Essential  minerals    124 

Essexite    152 

Extinction  angle. 42,  64,  118,  119 
Extraordinary  ray...  18,  22,  42 
Extrusive  flow : 127 

Farrell    185 

Fault  breccia   173 

Feldspar    43 

Felsitic  texture   126 

Flint 176 

Fluorite 74 

Gabbro    150 

Garnet   72 

Garnierite  155 

Gases,  influence  of 132 

Geological  mapping   185 

Geological  observations   .  .  .  185 

Glass    159 

Glassy  texture 126,  128 

Gneiss    180 

Granite    138 

Granitoid  texture 127 

Graphic   granite    140 

Graywacke    174 

Greisen    140 

Gypsum   175 

Halite 176 

Hematite 77 

Hexagonal  minerals    41,  77 

Holocrystalline     128 

Hornblende   103 

Hornblendite    157 

Hornfels    142 

Hypersthene  90 

Hypidiomorphic   128 

Hypocrystalline    128 


INDEX 


195 


Iceland  spar 22 

Iddings    63 

Idiomorphic     128 

Igneous  rocks  ....  123,  137,  138 

Ijolite 154 

Ilmenite 78 

Immersion  method    38 

Index  of  refraction 15,  36 

Interference  21,  42,  44,- 45,  48,  50 

Interference   figures    51-56 

Intrusive  127 

Isometric  minerals.  14,  40,69,70 

Isotropic  media   13 

Kaolinite 112 

Kilauea    167 

Kilimanjaro    163 

Kimberlite    155 

La  Crpix  56 

Labradorite     121 

Lepidolite    107 

Leucite 72 

Leucite  basalt   169 

Leucite  phonolite 162 

Leucite  syenite    147 

Leucitite    169 

Light,  nature  of   13 

Limburgite    169 

Limestone    176 

Liparite 158 

Lithographic   limestone 177 

Loess    175 

Magma    130 

Magmatic  stoping 131 

Magnetite    71 

Malignite    153 

Mannebach    114 

Mapping    185 

Marble 182 


Mauna  Loa 167 

Metamorphic  rocks .  123,  172,  177 

Metamorphism   177 

Mica    105 

Mica  plate 57,  59,  63 

Microcline  120 

Microcrystalline 128 

Micrometer  30 

Microscope 24 

Mineral    determination. . . . 

33-67,  133 

Mineral  description 68-122 

Mineralizers   126 

Mirror   28 

Missourite 154 

Monoclinic  minerals,  48,  96-119 
Muscovite    106 

Natrolite   94 

Negative  character 57-64 

Nephelite     83 

Nephelite  basalt    169 

Nephelite  syenite 147 

Newton's  color  scale 45 

Nicol  prism   21,  24 

Norite    150 

Objective     28 

Oblique  extinction 65 

Obsidian 159 

Obtuse  bisectrix 60 

Ocular    30 

Oligoclase    121 

Olivine  92,  155,  166 

Olivine  basalt 166 

Olivine  diabase   166 

Oolitic  limestone  177 

Opal   69 

Opaque  minerals    34,  41 

Ophicalcite    183 

Ophitic  texture    166 


196 


OPTICAL  MINERALOGY  AND  PETROGRAPHY 


Optic  axial  plane   49 

Optic  axis  26,  41,  51 

Optic  normal    49 

Optic  plane   49 

Optic  section    42 

Optical  character 56-64 

Optical  mineralogy '     9 

Orbicular  granite   141 

Order  of  color 47 

Ordinary  ray  18,  22,  42 

Orthoclase 119 

Orthorhombic   minerals . .  48,   86 

Pantellerite    160 

Parallel  extinction 65 

Pegmatite   139 

Penninite    109 

Pericline  twinning   114 

Peridotite    155 

Perlite   160 

Petrogeny    9,   130 

Petrography   9,  10,  123 

Petrology    9 

Phenocrysts    127 

Phlogopite    108 

Phonolite    135,  162 

Phosphate   177 

Phyllite    182 

Pitchstone    159 

Pleochroism   66,  67 

Plutonic  rocks 129,  138 

Polarization    21 

Polarizer 26,  41 

Polarizing  microscope 24 

Porphyritic  texture 127 

Positive  character 57-64 

Primary  minerals   125 

Principal   optic    section.  .26,   42 

Prospecting    185 

Pumice    159 

Pyrite    70 


Pyroclastic  rocks 170 

Pyroxene   89,  95 

Pyroxenite    157 

Pyrrhotite 70,  151 

Qualitative  classification  . .  123 
Quantitative  classification .  123 
Quarter  undulation  mica 

plate  57,  59,  63 

Quartz  44,  79,  134 

Quartzite  181 

Quartz-sensitive  tint  

58,  60,  61,  66 

Quartz  wedge 47,  59,  62 

Recrystallization    178,   181 

Refraction 15-18,  21,  36 

Relief   36,  40 

Rhyolite    135,  158 

Riebeckite    104 

Rock  classification    

Rogers    68 

Rosenbusch's  law 128 

Rutile    75 

Sanadine   120 

Sand  dunes 175 

Sandstone    174 

Scale  of  birefringence  ....  50 

Scale  of  refringence 40 

Scapolite   76 

Schist    181 

Secondary  minerals    125 

Sedimentary  rocks 123,  172 

Sericite    107 

Serpentine   88,  183 

Serpentine  rock  183 

Shale    174 

Shonkinite    153 

Siderite    81 

Slate    .                                    .  182 


INDEX 


197 


Soapstone    183 

Sodalite   73 

Spinel     71 

Stage    27 

Staurolite    87 

Steatite     183 

Syenite    145 

Talc    93 

Talus  breccia   173 

Tephrite    168 

Tetragonal  minerals   ....41,  75 

Texture 125,  128 

Theralite 153 

Titanite   112 

Topaz     87 

Total   reflection    16 

Tourmaline   84 

Trachydolerite    168 

Trachyte    161 

Trap     166 

Travertine     177 


Tremolite    102 

Triclinic  minerals  ..48,  120-122 

Tuff 171 

Twinning    35 

Undulatory  extinction   66 

Uniaxial   crystals    

14,  41,  51,  59,  01,  75 

Uralite    99 

Verde  antique    183 

Volcanic  agglomerate 170 

Volcanic  breccia   171,  174 

Volcanic  rocks    129,   158 

Wave-length    13,  21 

Werner,   A.    G 10 

Wernerite    76 

Winchell   40,  50 

Zircon    76 

Zirkel,  Ferdinand 11,  12 

Zoisite    .  .   Ill 


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